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Routing Convergence

August 25, 2014

by Matt Conran Blog

Routing Convergence

Routing convergence, a critical aspect of network performance, refers to the process of network routers exchanging information to update their routing tables in the event of network changes. It ensures efficient and reliable data transmission, minimizing disruptions and optimizing network performance. In this blog post, we will delve into the intricacies of routing convergence, exploring its importance, challenges, and best practices.

Routing convergence refers to the process by which a network's routing tables reach a consistent and stable state after making changes. It ensures that all routers within a network have up-to-date information about the available paths and can make efficient routing decisions.

When a change occurs in a network, such as a link failure or the addition of a new router, routing convergence is necessary to update the routing tables and ensure that packets are delivered correctly. The goal is to minimize the time it takes for all routers in the network to converge and resume normal routing operations.

Several mechanisms and protocols contribute to routing convergence. One of the critical components is the exchange of routing information between routers. This can be done through protocols such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF), or Border Gateway Protocol (BGP).

Matt Conran

Highlights: Routing Convergence

Understanding Convergence

Router convergence means they have the same topological information about the network they operate. To converge, a set of routers must have collected all topology information from each other using the routing protocol implemented. For this information to be accurate, it must reflect the current state of the network and not contradict other routers’ topology information.

All routers agree upon the topology of a converged network. For dynamic routing to work, a set of routers must be able to communicate with each other. All Interior Gateway Protocols depend on convergence. An autonomous system in operation is usually converged or convergent. Exterior Gateway Routing Protocol BGP rarely converges due to the size of the Internet.

Convergence Process

Each router in a routing protocol attempts to exchange topology information about the network. The extent, method, and type of information exchanged between routing protocols, such as BGP4, OSPF, and RIP, differs. A routing protocol convergence occurs once all routing protocol-specific information has been distributed to all routers. In the event of a routing table change in the network, convergence will be temporarily broken until the change has been successfully communicated to all routers.

Example: The convergence process

During the convergence process, routers exchange information about the network’s topology. Based on this information, they update their routing tables and calculate the most efficient paths to reach destination networks. This process continues until all routers have consistent and accurate routing tables.

The convergence time can vary depending on the size and complexity of the network and the routing protocols used. Convergence can happen relatively quickly in smaller networks, while more extensive networks may take longer to achieve convergence.

Network administrators can employ various strategies to optimize routing convergence. These include implementing fast convergence protocols, such as OSPF’s Fast Hello and Bidirectional Forwarding Detection (BFD), which minimize the time it takes to detect and respond to network changes.

Mechanisms for Achieving Routing Convergence:

1. Routing Protocols:

– Link-State Protocols: OSPF (Open Shortest Path First) and IS-IS (Intermediate System to Intermediate System) are examples of link-state protocols. They use flooding techniques to exchange information about network topology, allowing routers to calculate the shortest path to each destination.

– Distance-Vector Protocols: RIP (Routing Information Protocol) and EIGRP (Enhanced Interior Gateway Routing Protocol) are distance-vector protocols that use iterative algorithms to determine the best path based on distance metrics.

2. Fast Convergence Techniques:

– Triggered Updates: When a change occurs in network topology, routers immediately send updates to inform other routers about the change, reducing the convergence time.

– Route Flapping Detection: Route flapping occurs when a network route repeatedly becomes available and unavailable. By detecting and suppressing flapping routes, convergence time can be significantly improved.

– Convergence Optimization: Techniques like unequal-cost load balancing and route summarization help optimize routing convergence by distributing traffic across multiple paths and reducing the size of routing tables.

3. Redundancy and Resilience:

– Redundant Links: Multiple physical connections between routers increase network reliability and provide alternate paths in case of link failures.

– Virtual Router Redundancy Protocol (VRRP): VRRP allows multiple routers to act as a single virtual router, ensuring seamless failover in case of a primary router failure.

– Multi-Protocol Label Switching (MPLS): MPLS technology offers fast rerouting capabilities, enabling quick convergence in case of link or node failures.

Strategies for Achieving Optimal Routing Convergence

a. Enhanced Link-State Routing Protocol (EIGRP): EIGRP is a dynamic routing protocol that utilizes a Diffusing Update Algorithm (DUAL) to achieve fast convergence. By maintaining a backup route in case of link failures and employing triggered updates, EIGRP significantly reduces the time required for routing tables to converge.

b. Optimizing Routing Metrics: Carefully configuring routing metrics, such as bandwidth, delay, and reliability, can help achieve faster convergence. Assigning appropriate weights to these metrics ensures that routers quickly select the most efficient paths, leading to improved convergence times.

c. Implementing Route Summarization: Route summarization involves aggregating multiple network routes into a single summarized route. This technique reduces the size of routing tables and minimizes the complexity of route calculations, resulting in faster converge

BGP Next Hop Tracking

BGP next hop refers to the IP address of the next router in the path towards a destination network. It serves as crucial information for routers to make forwarding decisions. Typically, BGP relies on the reachability of the next hop to determine the best path. However, various factors can affect this reachability, including link failures, network congestion, or misconfigurations. This is where BGP next hop tracking comes into play.

By incorporating next hop tracking into BGP, network administrators gain valuable insights into the reachability status of next hops. This information enables more informed decision-making regarding routing policies and traffic engineering. With real-time tracking, administrators can dynamically adjust routing paths based on the availability and quality of next hops, leading to improved network performance and reliability.

Benefits of Efficient Routing Convergence:

1. Improved Network Performance: Efficient routing convergence reduces network congestion, latency, and packet loss, improving overall network performance.

2. Enhanced Reliability: Routing convergence ensures uninterrupted communication and minimizes downtime by quickly adapting to changes in network conditions.

3. Scalability: Proper routing convergence techniques facilitate network expansion and accommodate increased traffic demands without sacrificing performance or reliability.

Example Technology: BFD

Bidirectional Forwarding Detection (BFD) is a lightweight protocol designed to detect failures in communication paths between routers or switches. It operates independently of the routing protocols and detects rapid failure by utilizing fast packet exchanges. Unlike traditional methods like hello packets, BFD offers sub-second detection, allowing quicker convergence and network stability. BFD is pivotal in achieving fast routing convergence, providing real-time detection, and facilitating swift rerouting decisions.

-Enhanced Network Resilience: By swiftly detecting link failures, BFD enables routers to act immediately, rerouting traffic through alternate paths. This proactive approach ensures minimal disruption and enhances network resilience, especially in environments where redundancy is critical.

-Reduced Convergence Time: BFD’s ability to detect failures within milliseconds significantly reduces the time required for converging routing protocols. This translates into improved network responsiveness, reduced packet loss, and enhanced user experience.

-Scalability and Flexibility: BFD can be implemented across various network topologies and routing protocols, making it a versatile solution. Whether a small enterprise network or a large-scale service provider environment, BFD adapts seamlessly, providing consistent performance and stability.

Convergence Time

Convergence time measures the speed at which a group of routers converges. Fast and reliable convergent routers are a significant performance indicator for routing protocols. The size of the network is also essential. A more extensive network will converge more slowly than a smaller one.

When a few routers are connected to RIP, a routing protocol that converges slowly, it can take several minutes for the network to converge. A triggered update for a new route can speed up RIP’s convergence, but a hold-down timer will slow flushing an existing route. OSPF is an example of a fast-convergence routing protocol. It is impossible to limit the speed at which OSPF routers can converge.

Unless specific hardware and configuration conditions are met, networks can never converge. “Flapping” interfaces (ones that frequently change between “up” and “down”) propagate conflicting information throughout the network, so routers cannot agree on the current state. Route aggregation can deprive certain parts of a network of detailed routing information, resulting in faster convergence of topological information.

Topological information

A set of routers in a network share the same topological information during convergence or routing convergence. Routing protocols exchange topology information between routers in a network. Routers in a network receive routing information when convergence is reached. Therefore, all routers know the network topology and optimal route in a converged network.

Any change in the network – for example, the failure of a device – affects convergence until all routers are informed of the change. The convergence time in a network is the time it takes for routers to achieve convergence after a topology change. In high-performance service provider networks, sensitive applications are run that require fast failover in case of failures. Several factors determine a network’s convergence rate:

Devices detect route failures. Finding a new forwarding path begins with identifying the failed device. The existence of virtual networks establishes device reachability through their longevity, as opposed to physical networks, in which events determine device availability. To achieve fast network convergence, the detection time – the time it takes to detect a failure – must be kept within acceptable limits.
In the event of a device failure on the primary route, traffic is diverted to the backup route. The failure or topology change has not yet affected all devices.
Routing protocols are said to achieve global repair or network convergence when they propagate a change in topology to all network devices.

Understanding UDLD

UDLD, at its core, is a layer 2 protocol designed to detect and mitigate unidirectional links in Ethernet connections. It actively monitors the link status, allowing network devices to promptly detect and address potential issues. By verifying the bidirectional communication between neighboring devices, UDLD acts as a guardian, preventing one-way communication breakdowns.

Implementing UDLD brings forth numerous advantages for network administrators and organizations alike.

Firstly, it enhances network reliability by identifying and resolving unidirectional link failures that could otherwise lead to data loss and network disruptions.

Secondly, UDLD helps troubleshoot by providing valuable insights into link quality and integrity. This proactive approach aids in reducing downtime and improving overall network performance.

Enhancing Routing Convergence

Network administrators can implement various strategies to improve routing convergence. One approach is to utilize route summarization, which reduces the number of routes advertised and processed by routers. This helps minimize the impact of changes in specific network segments on overall convergence time.

Furthermore, implementing fast link failure detection mechanisms, such as Bidirectional Forwarding Detection (BFD), can significantly reduce convergence time. BFD allows routers to quickly detect link failures and trigger immediate updates to routing tables, ensuring faster convergence.

Factors Influencing Routing Convergence

Several factors impact routing convergence in a network. Firstly, the efficiency of the routing protocols being used plays a crucial role. Protocols such as OSPF (Open Shortest Path First) and EIGRP (Enhanced Interior Gateway Routing Protocol) are designed to facilitate fast convergence by quickly adapting to network changes.

Additionally, network topology and scale can affect routing convergence. Large networks with complex topologies may require more time for routers to converge due to the increased number of routes and potential link failures. Network administrators must carefully design and optimize the network architecture to minimize convergence time.

Control and data plane

When considering routing convergence with forwarding routing protocols, we must first highlight that a networking device is tasked with two planes of operation—the control plane and the data plane. The job of the data plane is to switch traffic across the router’s interfaces as fast as possible, i.e., move packets. The control plane has the more complex operation of putting together and creating the controls so the data plane can operate efficiently. How these two planes interact will affect network convergence time.

The network’s control plane finds the best path for routing convergence from any source to any network destination. For quick convergence routing, it must react quickly and dynamically to changes in the network, both on the LAN and on the WAN.

Monitoring and Troubleshooting Routing Convergence

Network administrators must monitor routing convergence to identify and promptly address potential issues. Network management tools, such as SNMP (Simple Network Management Protocol) and NetFlow analysis, can provide valuable insights into routing convergence performance, including convergence time, route flapping, and stability.

When troubleshooting routing convergence problems, administrators should carefully analyze routing table updates, link state information, and routing protocol logs. This information can help pinpoint the root cause of convergence delays or inconsistencies, allowing for targeted remediation.

Related: For pre-information, you may find the following posts helpful:

Routing Convergence

Convergence Time Definition.

I found two similar definitions of convergence time:

“Convergence is the amount of time ( and thus packet loss ) after a failure in the network and before the network settles into a steady state.” Also, ” Convergence is the amount of time ( and thus packet loss) after a failure in the network and before the network responds to the failure.”

The difference between the two convergence time definitions is subtle but essential – steady-state vs. just responding. The control plane and its reaction to topology changes can be separated into four parts below. Each area must be addressed individually, as leaving one area out results in slow network convergence time and application time-out.

IP routing

Moving IP Packets

A router’s primary role is moving an IP packet from one network to another. Routers select the best loop-free path in a network to forward a packet to its destination IP address. A router learns about nonattached networks through static configuration or dynamic IP routing protocols. Both static and dynamic are examples of routing protocols.

With dynamic IP routing protocols, we can handle network topology changes dynamically. Here, we can distribute network topology information between routers in the network. When there is a change in the network topology, the dynamic routing protocol provides updates without intervention when a topology change occurs.

On the other hand, we have IP routing to static routes, which do not accommodate topology changes very well and can be a burden depending on the network size. However, static routing is a viable solution for minimal networks with no modifications.

Diagram: Dynamic Routing Protocols. Source Cisco Press.

Knowledge Check: Bidirectional Forwarding Detection

Understanding BFD

BFD is a lightweight protocol designed to detect faults in the forwarding path between network devices. It operates at a low level, constantly monitoring the connectivity and responsiveness of neighboring devices. BFD can quickly detect failures by exchanging control packets and taking appropriate action to maintain network stability.

The Benefits of BFD

The implementation of BFD brings numerous advantages to network administrators and operators. Firstly, it provides rapid fault detection, reducing downtime and minimizing the impact of network failures. Additionally, BFD offers scalable and efficient operation, as it consumes minimal network resources. This makes it an ideal choice for large-scale networks where resource optimization is crucial.

BFD runs independently from other (routing) protocols. Once it’s up and running, you can configure protocols like OSPF, EIGRP, BGP, HSRP, MPLS LDP, etc., to use BFD for link failure detection instead of their mechanisms. When the link fails, BFD informs the protocol. When BFD no longer receives its control packets, it realizes we have a link failure and reports this to OSPF. OSPF will then tear down the neighbor adjacency.

Use Cases of BFD

BFD finds its applications in various networking scenarios. One prominent use case is link aggregation, where BFD helps detect link failures and ensures seamless failover to alternate links. BFD is also widely utilized in Virtual Private Networks (VPNs) to monitor the connectivity of tunnel endpoints, enabling quick detection of connectivity issues and swift rerouting.

Implementing BFD in Practice

Implementing BFD requires careful consideration and configuration. Network devices must be appropriately configured to enable BFD sessions and define appropriate parameters such as timers and thresholds. Additionally, network administrators must ensure proper integration with underlying routing protocols to maximize BFD’s efficiency.

Convergence Routing and Network Convergence Time

Network convergence connects multiple computer systems, networks, or components to establish communication and efficient data transfer. However, it can be a slow process, depending on the size and complexity of the network, the amount of data that needs to be transferred, and the speed of the underlying technologies.

For networks to converge, all of the components must interact with each other and establish rules for data transfer. This process requires that the various components communicate with each other and usually involves exchanging configuration data to ensure that all components use the same protocols.
Network convergence is also dependent on the speed of the underlying technologies.

To speed up convergence, administrators should use the latest technologies, minimize the amount of data that needs to be transferred, and ensure that all components are correctly configured to be compatible. By following these steps, network convergence can be made faster and more efficient.

Example: OSPF

To put it simply, convergence or routing convergence is a state in which a set of routers in a network share the same topological information. For example, we have ten routers in one OSFP area. OSPF is an example of a fast-converging routing protocol. A network of a few OSPF routers can converge in seconds.

The routers within the OSPF area in the network collect the topology information from one another through the routing protocol. Depending on the routing protocol used to collect the data, the routers in the same network should have identical copies of routing information.

Different routing protocols will have additional convergence time. The time the routers take to reach convergence after a change in topology is termed convergence time. Fast network convergence and fast failover are critical factors in network performance. Before we get into the details of routing convergence, let us recap how networking works.

network convergence time — Diagram: Network convergence time.

Unlike IS-IS, OSPF has fewer “knobs” for optimizing convergence. This is probably because IS-IS is being developed and supported by a separate team geared towards ISPs, where fast convergence is a competitive advantage.

Diagram: Example Convergence Time with OSPF. Source INE.

OSPF: Incremental SPF

OSPF calculates the SPT (Shortest Path Tree) using the SPF (Shortest Path First) algorithm. SPTs are built by OSPF routers within the same area with the same LSAs, LSDBs, and LSAs. OSPF routers will rerun a full SPF calculation even when there is just a single change in the network topology (change to an LSA type 1 and LSA type 2).

If a topology change occurs, we should run a full SPT calculation to find the shortest paths to all destinations. Unfortunately, we also calculate paths that have not changed since the last SPF.

In incremental SPF, OSPF only recalculates the parts of the SPT that have changed.

Because you don’t run a full SPF all the time, the router’s CPU load decreases,, and convergence times improve—additionally, your router stores the previous SPT copy, which requires more memory.

In three scenarios, incremental SPF is beneficial:

Adding (or removing) a leaf node to a branch
Link failure in non-SPT
Link failure in a branch of SPT

When many routers are in a single area, and the CPU load is high because of OSPF, incremental SPF can be enabled per router.

Forwarding Paradigms

We have bridging routing and switching with data and the control plane. So, we need to get packets across a network, which is easy if we have a single cable. You need to find the node’s address, and small and non-IP protocols would use a broadcast. When devices in the middle break this path, we can use source routing, path-based forwarding, and hop-by-hop address-based forwarding based solely on the destination address.

When protocols like IP came into play, hop-by-hop destination-based forwarding became the most popular; this is how IP forwarding works. Everyone in the path makes independent forwarding decisions. Each device looks at the destination address, examines its lookup tables, and decides where to send the packet.

Finding paths across the network

How do we find a path across the network? We know there are three ways to get packets across the network – source routing, path-based forwarding, and hop-by-hop destination-based forwarding. So, we need some way to populate the forwarding tables. You need to know how your neighbors are and who your endpoints are. This can be static routing, but it is more likely to be a routing protocol. Routing protocols have to solve and describe the routing convergence on the network at a high level.

When we are up and running, events can happen to the topology that force or make the routing protocols react and perform a convergence routing state. For example, we have a link failure, and the topology has changed, impacting our forwarding information. So, we must propagate the information and adjust the path information after the topology change. We know these convergence routing states are to detect, describe, switch, and find.

Rouitng Convergence	Convergence
Detect
Describe
Switch
Find

To better understand routing convergence, I would like to share the network convergence time for each routing protocol before diving into each step. The times displayed below are from a Cisco Live session based on real-world case studies and field research. We are separating each of the convergence routing steps described above into the following fields: Detect, describe, find alternative, and total time.

Routing Protocol	RIP	OSPF	EIGRP
Detect	<1 second-best, 105 seconds average	<1 second-best, 20 seconds average	<1 second-best, 15 seconds average.30 seconds worst
Describe	15 seconds average, 30 seconds worst	1 second-best, 5 seconds average.	2 seconds
Find Alternative	15 seconds average, 30 seconds worst	1-second average.	*** <500ms per query hop average Assume a 2-second average
Total Time	Best Average Case: 31 seconds Average Case: 135 seconds Worse Case: 179 seconds	Best Average Case: 2 to 3 seconds Average Case: 25 seconds Worse Case: 45 seconds	Best Average Case: <1 second Average Case: 20 seconds Worse Case: 35 seconds

*** The alternate route is found before the described phase due to the feasible successor design with EIGRP path selection.

Convergence Routing

Convergence routing: EIGPR

EIGRP is the fastest but only fractional. EIGRP has a pre-built loop-free path known as a feasible successor. The FS route has a higher metric than the successor, making it a backup route to the successor route. The effect of a pre-computed backup route on convergence is that EIGRP can react locally to a change in the network topology; nowadays, this is usually done in the FIB. EIGRP would have to query for the alternative route without a feasible successor, increasing convergence time.

However, you can have a Loop Free Alternative ( LFA ) for OSPF, which can have a pre-computed alternate path. Still, LFAs can only work with specific typologies and don’t guarantee against micro-loops ( EIGRP guarantees against micro-loops).

Lab Guide: EIGRP LFA FRR

With Loop-Free Alternate (LFA) Fast Reroute (FRR), EIGRP can switch to a backup path in less than 50 milliseconds. Fast rerouting means switching to another next hop, and a loop-free alternate refers to a loop-free alternative path.

Perhaps this sounds familiar to you. After all, EIGRP has feasible successors. The alternate paths calculated by EIGRP are loop-free. As soon as the successor fails, EIGRP can use a feasible successor.

It’s true, but there’s one big catch. In the routing table, EIGRP feasible successors are not immediately installed. Only one route is installed, the successor route. EIGRP installs the feasible successor when the successor fails, which takes time. By installing both successor routes and feasible successor routes in the routing table, fast rerouting makes convergence even faster.

These four routers run EIGRP; there’s a loopback on R4 with network 4.4.4.4/32. R1 can go through R2 or R3 to get there. The delay on R1’s GigabitEthernet3 interface has increased, so R2 is our successor, and R3 is our feasible successor. The output below is interesting. We still see the successor route, but at the bottom, you can see the repair path…that’s our feasible successor.

TCP Congestion control

Ask yourself, is < 1-second convergence fast enough for today’s applications? Indeed, the answer would be yes for some non-critical applications that work on TCP. TCP has built-in backoff algorithms that can deal with packet loss by re-transmitting to recover lost segments. However, non-bulk data applications like video and VOIP have stricter rules and require fast convergence and minimal packet loss.

For example, a 5-second delay in routing protocol convergence could mean several hundred dropped voice calls. A 50-second delay in a Gigabit Ethernet link implies about 6.25 GB of lost information.

Adding Resilience

To add resilience to a network, you can aim to make the network redundant. When you add redundancy, you are betting that outages of the original path and the backup path will not co-occur and that the primary path does not fate share with the backup path ( they do not share common underlying infrastructure, i.e., physical conducts or power ).

There needs to be a limit on the number of links you add to make your network redundant, and adding 50 extra links does not make your network 50 times more redundant. It does the opposite! The control plane is tasked with finding the best path and must react to modifications in the network as quickly as possible.

However, every additional link you add slows down the convergence of the router’s control plane as there is additional information to compute, resulting in longer convergence times. The correct number of backup links is a trade-off between redundancy versus availability. The optimal level of redundancy between two points should be two or three links. The fourth link would make the network converge slower.

Convergence Routing — Diagram: Convergence routing and adding resilience.

Routing Convergence and Routing Protocol Algorithms

Routing protocol algorithms can be tweaked to exponentially back off and deal with bulk information. However, no matter how many timers you use, the more data in the routing databases, the longer the convergence times. The primary way to reduce network convergence is to reduce the size of your routing tables by accepting just a default route, creating a flooding boundary domain, or using some other configuration method.

For example, a common approach in OSPF to reduce the size of routing tables and flooding boundaries is to create OSPF stub areas. OSPF stub areas limit the amount of information in the area. For example, EIGRP limits the flooding query domain by creating EIGRP stub routers and intelligently designing aggregation points. Now let us revisit the components of routing convergence:

Routing Convergence Step	Routing Convergence Details
Step 1	Failure detection
Step 2	Failure propagation ( flooding, etc.) IGP Reaction
Step 3	Topology/Routing calculation. IGP Reaction.
Step 4	Update the routing and forwarding table ( RIB & FIB)

Stage 1: Failure Detection

The first and foremost problem facing the control plane is quickly detecting topology changes. Detecting the failure is the most critical and challenging part of network convergence. It can occur at different layers of the OSI stack – Physical Layers ( Layer 1), Data Link Layer ( Layer 2 ), Network Layer ( Layer 3 ), and Application layer ( Layer 7 ). There are many types of techniques used to detect link failures, but they all generally come down to two basic types:

Event-driven notification occurs when a carrier is lost or when one network element detects a failure and notifies the other network elements.
Polling-driven notification – generally HELLO protocols that test the path for reachability, such as Bidirectional Forwarding Detection ( BFD ). Event-driven notifications are always preferred over polling-driven ones as the latter have to wait for three polls before declaring a path down. However, there are some cases when you have multiple Layer devices in the path, and HELLO polling systems are the only method that can be used to detect a failure.

Layer 1 failure detection

Layer 1: Ethernet mechanisms like auto-negotiation ( 1 GigE ) and link fault signaling ( 10 GigE 802.3ae/ 40 GigE 802.3ba ) can signal local failures to the remote end.

However, the challenge is getting the signal across an optical cloud, as relaying the fault information to the other end is impossible. When there is a “bump” in the Layer 1 link, it is not always possible for the remote end to detect the failure. In this case, the link fault signaling from Ethernet would get lost in the service provider’s network.

The actual link-down / interface-down event detection is hardware-dependent. Older platforms, such as the 6704 line cards for the Catalyst 6500, used a per-port polling mechanism, resulting in a 1 sec detect link failure period. More recent Nexus switches and the latest Catalyst 5600 line cards have an interrupt-driven notification mechanism resulting in high-speed and predictable link failure detection.

Layer 2 failure detection

Layer 2: The layer 2 detection mechanism will kick in if the Layer 1 mechanism does not. Unidirectional Link Detection ( UDLD ) is a Cisco proprietary lightweight Layer 2 failure detection protocol designed for detecting one-way connections due to physical or soft failure and miss-wirings.

A key point: UDLD is a slow protocol

UDLD is a reasonably slow protocol that uses an average of 15 seconds for message interval and 21 seconds for detection. Its period has raised questions about its use in today’s data centers. However, the chances of miswirings are minimal; Layer 1 mechanisms always communicate unidirectional physical failure, and STP Bridge Assurance takes care of soft failures in either direction.

STP Bridge assurance turns STP into a bidirectional protocol and ensures that the spanning tree never fails to open and only fails to close. Failing open means that if a switch does not hear from its neighbor, it immediately starts forwarding on initially blocked ports, causing network havoc.

Layer 3 failure detection

Layer 3: In some cases, failure detection has to reply to HELLO protocols at Layer 3. This is needed when there are intermediate Layer 2 hops over Layer links and concerns over uni-direction failures on point-to-point physical links arise.

All Layer 3 protocols use HELLOs to maintain neighbor adjacency and a DEAD time to declare a neighbor dead. These timers can be tuned for faster convergence. However, it is generally not recommended due to the increase in CPU utilization causing false positives and the challenges ISSU and SSO face. They enable bidirectional forwarding detection ( BFD ) as the layer 3 detection mechanism is strongly recommended over aggressive protocol times, and they use BFD for all protocols.

Bidirectional Forwarding Detection ( BFD ) is a lightweight hello protocol for sub-second Layer 3 failure detection. It can run over multiple transport protocols, such as MPLS, THRILL, IPv6, and IPv4, making it the preferred method.

Stage 2: Routing convergence and failure propagation

When a change occurs in the network topology, it must be registered with the local router and transmitted throughout the rest of the network. The transmission of the change information will be carried out differently for Link-State and Distance Vector protocols. Link state protocols must flood information to every device in the network, and the distance vector must process the topology change at every hop through the network.

The processing of information at every hop may lead you to conclude that link-state protocols always converge more quickly than path-vector protocols, but this is not the case. EIGRP, due to its pre-computed backup path, will converge more rapidly than any link-state protocol.

Diagram: Routing convergence and failure propagation

To propagate topology changes as quickly as possible, OSPF ( Link state ) can group changes into a few LSA while slowing down the rate at which information is flooded, i.e., do not flood on every change. This is accomplished with link-state flood timer tuning combined with exponential backoff systems, such as link-state advertisement delay / initial link-state advertisement throttle delay.

Unfortunately, Distance Vector Protocols do not have such timers. Therefore, reducing the routing table size is the only option for EIGRP. This can be done by aggregating and filtering reachability information ( summary route or Stub areas ).

Stage 3: Topology/Routing calculation

Similar to the second step, link-state protocols use exponential back-off timers in this step. These timers adjust the waiting time for OSPF and ISIS to wait after receiving new topology information before calculating the best path.

Stage 4: Update the routing and forwarding table ( RIB & FIB)

Finally, after the topology information has been flooding through the network and a new best path has been calculated, the new best path must be installed in the Forwarding Information Base ( FIB ). The FIB is a copy of the RIB in hardware, and the forwarding process finds it much easier to read than the RIB. However, again, this is usually done in hardware. Most vendors offer features that will install a pre-computed backup path on the line cards forwarding table so the fail-over from the primary path to the backup path can be done milliseconds without interrupting the router CPU.

Closing Points: Routing Convergence

Routing convergence refers to the process by which network routers exchange routing information and adapt to changes in network topology or routing policies. It involves the timely update and synchronization of routing tables across the network, allowing routers to determine the best paths for forwarding data packets.

Routing convergence is vital for maintaining network stability and minimizing disruptions. Network traffic may experience delays, bottlenecks, or even failures without proper convergence. Routing convergence enables efficient and reliable communication by ensuring all network routers have consistent routing information.

Summary: Routing Convergence

Routing convergence is crucial in network management, ensuring smooth and efficient communication between devices. In this blog post, we will explore the concept of routing convergence, its importance in network operations, common challenges faced, and strategies to achieve faster convergence times.

Section 1: Understanding Routing Convergence

Routing convergence refers to network protocols adapting to changes in network topology, such as link failures or changes in network configurations. It involves recalculating and updating routing tables to ensure the most optimal paths for data transmission. Network downtime can be minimized by achieving convergence quickly, and data can flow seamlessly.

Section 2: The Importance of Fast Convergence

Fast routing convergence is critical for maintaining network stability and minimizing disruptions. In today’s fast-paced digital landscape, where businesses rely heavily on uninterrupted connectivity, delays in convergence can result in significant financial losses, degraded user experience, and even security vulnerabilities. Therefore, network administrators must prioritize measures to enhance convergence speed.

Section 3: Challenges in Routing Convergence

While routing convergence is essential, it comes with its challenges. Network size, complex topologies, and diverse routing protocols can significantly impact convergence times. Additionally, suboptimal route selection, route flapping, and inefficient link failure detection can further hinder the convergence process. Understanding these challenges is crucial for devising practical solutions.

Section 4: Strategies for Achieving Faster Convergence

To optimize routing convergence, network administrators can implement various strategies. These include:

1. Implementing Fast Convergence Protocols: Utilizing protocols like Bidirectional Forwarding Detection (BFD) and Link State Tracking (LST) can expedite the detection of link failures and trigger faster convergence.

2. Load Balancing and Redundancy: Distributing traffic across multiple paths and employing redundancy mechanisms, such as Equal-Cost Multipath (ECMP) routing, can mitigate the impact of link failures and improve convergence times.

3. Optimizing Routing Protocol Parameters: Fine-tuning routing protocol timers, hello intervals, and dead intervals can contribute to faster convergence by reducing the time it takes to detect and react to network changes.

Section 5: Conclusion

In conclusion, routing convergence is fundamental to network management, ensuring efficient data transmission and minimizing disruptions. By understanding the concept, recognizing the importance of fast convergence, and implementing appropriate strategies, network administrators can enhance network stability, improve user experience, and safeguard against potential financial and security risks.

Forwarding Routing Protocols

August 10, 2014

by Matt Conran Blog

Forwarding Routing Protocols

Forwarding routing protocols are crucial for computer networks, enabling efficient data transmission and device communication. This blog post will explore forwarding routing protocols, their significance, and some famous examples.

Forwarding routing protocols, or routing algorithms, determine the paths data packets take in a network. These protocols are vital in delivering information from a source to a destination device. They ensure data packets are transmitted along the most efficient paths, minimizing delays and optimizing network performance.

Forwarding routing protocols are essential components of network communication. They determine the best path for data packets to travel from source to destination, taking into consideration factors such as network congestion, link reliability, and available bandwidth. By efficiently directing traffic, forwarding routing protocols enhance network performance and ensure reliable data transmission.

There are several types of forwarding routing protocols, each with its own characteristics and use cases. This section will explore some of the most common ones, including:

- Distance Vector Routing Protocols:
- Link State Routing Protocols
- Hybrid Routing Protocols:

Choosing the right forwarding routing protocol for a specific network environment requires careful consideration of various factors.

What is IP routing? To answer this question, we must first understand routers' protocol to forward messages. Forwarding routing protocols are networking protocols that facilitate communication between different network nodes.

They are responsible for finding the optimal path for data to travel from one node to another and managing and maintaining routing tables containing information about the available paths for various destinations.

Matt Conran

Highlights: Forwarding Routing Protocols

Routing Protocols

Protocol Algorithms:

Forwarding routing protocols are algorithms and protocols used by routers to determine the best path for routing data packets from source to destination. These protocols use routing tables and various metrics to make intelligent decisions about packet forwarding. They facilitate efficient data transmission by dynamically updating and maintaining routing information.

Forwarding routing protocols are vital for maintaining a reliable and efficient network infrastructure. They enable routers to make intelligent routing decisions, adapt to network topology changes, and handle traffic load balancing. Without forwarding routing protocols, networks would be inefficient, prone to congestion, and lack fault tolerance.

**Types of Forwarding Routing Protocols**

Routing protocols can be broadly categorized into different types, each with unique characteristics and applications. Some of the most common types include:

1. **Distance Vector Protocols**: These protocols, such as RIP (Routing Information Protocol), determine the best path based on the distance to the destination. They are simple but may struggle with larger networks due to slower convergence times.

2. **Link State Protocols**: Protocols like OSPF (Open Shortest Path First) and IS-IS (Intermediate System to Intermediate System) fall under this category. They maintain a complete map of the network topology, allowing for faster convergence and more efficient routing decisions.

3. **Path Vector Protocols**: BGP (Border Gateway Protocol) is a prime example. Used extensively on the internet, BGP is crucial for inter-domain routing, providing robust scalability and policy-based routing.

**Challenges in Implementing Routing Protocols**

Despite their advantages, implementing routing protocols comes with its set of challenges. Network administrators must consider factors like scalability, security, and protocol compatibility. Ensuring that routing decisions are both efficient and secure requires constant monitoring and adjustment. Moreover, with the ever-increasing complexity of networks, the need for more sophisticated and adaptable routing algorithms is more pressing than ever.

Example: Understanding RIP

RIP, or Routing Information Protocol, is one of the oldest distance-vector routing protocols that is still in use today. Initially developed for smaller networks, RIP operates by sharing routing information between neighboring routers. It uses the hop count as its metric, representing the number of routers a packet must traverse to reach its destination.

RIP employs a simple approach to routing. Each router periodically broadcasts its routing table to its neighboring routers, which, in turn, update their tables accordingly. This process ensures that every router within the network has up-to-date knowledge of the available routes.

Example: OSPFv3

Understanding OSPFv3

OSPFv3, which stands for Open Shortest Path First version 3, is an extension of OSPF tailored to support IPv6 networks. It operates on the same basic principles as OSPF, such as link-state advertisements and the shortest path first algorithm. However, OSPFv3 introduces several vital enhancements and modifications to accommodate IPv6 addressing and network infrastructure differences.

One of the significant changes in OSPFv3 is the handling of IPv6 addresses. Unlike OSPF for IPv4, OSPFv3 does not rely on network masks for route determination. Instead, It utilizes the IPv6 prefix length information embedded in each IPv6 address. This simplifies the routing process and ensures compatibility with IPv6’s expanded address space.

Starting points: Networking Protocols

Networking protocols facilitate communication between computer systems. Today, computer systems use three main protocols: Ethernet, TCP/IP, and Fibre Channel. Cables are used to connect various networking devices using Ethernet. Wireless computer networks are created using the TCP/IP protocol. Fiber channels are used to transfer large amounts of data between computers.

Routing, forwarding, and switching are network terms used when data is sent from one party to another. Each plays a crucial role in data delivery. Routing is the process of moving data from one device to another. Forwarding involves collecting data from one device and sending it to another. With switching, data is collected from one device and sent to multiple devices based on their MAC addresses.

Moving data between devices

Moving data between devices is known as routing. Networking devices called routers perform routing most of the time. Furthermore, routers can forward connections to other networks. In addition, routers help create and manage networks. Within networks, they move data from one device to another. Routers can also transmit data across different networks in some cases. Routing is done at the network layer in the OSI model. The network layer chooses the optimal or shortest path from sender to receiver. Optimal paths are calculated using routing algorithms.

The forwarding process involves collecting data from one device and sending it to another. Unlike routing, this process does not move data between devices. Forwarding differs from routing because it performs some actions instead of simply forwarding packets. It doesn’t decide the path. The packets are only sent to another network in the forwarding process: The network layer performs routing and forwarding. Forwarding devices collect data and send it to another device. Switches, routers, and hubs are standard forwarding devices.

Forwarding Methods

Let’s discuss some popular forwarding methods in networking. In the next hop method, packets are sent from the router to the next gateway in the direction of the destination. Routing tables with network-specific entries contain destinations connected to routers. A routing table is a set of rules, often displayed as a table, that determine where data packets will be directed over an Internet Protocol (IP) network. Routers and switches, as well as all IP-enabled devices, use routing tables. Lastly, when using the host-specific method, the routing table contains information about all the destination hosts in the destination network.

The Role of Switching

Data is switched from one port to another by collecting it from one port and sending it to the destination by switching. There are two types of switching: connectionless and connection-oriented. Connectionless switching does not require handshaking to establish a connection. A forwarding table determines how packets received in a port are sent. Conversely, connection-oriented switching uses a predefined circuit between the sender and receiver and an intermediate node ID.

Switching techniques can be divided into circuit, message, and packet switching. Circuit switching requires establishing a circuit before sending data. The received data is treated as a message when message switching is used and sent to the intermediate switching device. Packet switching breaks the data into small chunks called packets. Each packet is transmitted independently.

IP Routing

Routing is the process of moving IP packets from one network to another. IP routing protocols or static configuration allow routers to learn about nonattached networks. When a network topology change occurs, dynamic IP routing protocols update the network topology without intervention. Depending on the size of the network, IP routing may be limited to static routes due to design or hardware limitations.

Static routes are not accommodating when the topology changes and can be burdensome for network engineers. An IP packet is forwarded to its destination IP address with the help of a router that selects a loop-free path through a network.

Autonomous systems are networks of interconnected routers and related systems managed by a common network administrator. A global network of autonomous systems makes up the Internet.

Rules and Algorithms

Forwarding routing protocols are rules and algorithms determining the best path for data packets to follow within a network. They facilitate the exchange of routing information between routers and ensure that information is forwarded most efficiently. These protocols direct data packets from the source device to the correct destination device, providing reliable and timely delivery.

Example: EIGRP DUAL

DUAL, an abbreviation for Diffusing Update Algorithm, is the decision-making process EIGRP routers use to calculate the best path to reach a destination. It ensures loop-free and efficient routing within a network. To comprehend DUAL, we must explore its key components: the feasible distance (FD), reported distance (RD), and successor and feasible successor routes.

Feasible Distance (FD) is the metric for the best-known path to a destination. It represents the cumulative cost of all the links on that path. Reported Distance (RD) is the metric for a neighbor’s path to the same destination. These two values play a vital role in DUAL’s decision-making process. Successor routes are the best paths chosen by DUAL to reach a destination. A router selects the path with the lowest FD as its successor route.

Feasible Successor routes, on the other hand, are backup paths that have a higher FD but are still loop-free. These routes are pre-calculated and provide fast convergence if the successor route fails. Network convergence refers to the time it takes routers to update their routing tables after a change occurs in the network topology. DUAL plays a crucial role in achieving rapid convergence in EIGRP. DUAL minimizes the time and resources required for network convergence by maintaining successor and feasible successor routes.

Example: MP-BGP with IPv6 Prefixes

Understanding MP-BGP

MP-BGP, or Multiprotocol Border Gateway Protocol, is a routing protocol that enables the exchange of routing information between networks. Unlike traditional BGP, which primarily deals with IPv4 routes, MP-BGP extends its capabilities to support multiple protocols, including IPv6. MP-BGP provides a flexible and scalable solution for modern network architectures by accommodating various protocols.

IPv6 adjacency refers to establishing connections between neighboring routers that utilize IPv6 addresses. With the depletion of available IPv4 addresses, IPv6 has become increasingly important in ensuring the continued growth and expansion of networks. MP-BGP plays a crucial role in enabling IPv6 adjacency by allowing routers to exchange routing information and establish connectivity efficiently and seamlessly.

Common Forwarding Routing Protocols

Two of the most commonly used forwarding routing protocols are Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP). OSPF is an interior gateway protocol (IGP) used within autonomous systems and networks managed by a single administrative entity. It uses a link-state algorithm to determine the best route for data to travel. Conversely, BGP is an exterior gateway protocol (EGP) used to connect autonomous systems. It uses a path vector algorithm to determine the best route for data to travel.

Both protocols are essential for routing data across networks, and they both have their advantages and disadvantages. OSPF is more efficient and supports more features, while BGP is more secure and reliable. However, both protocols are required to communicate data across networks efficiently.

Understanding MPLS Forwarding

MPLS (Multiprotocol Label Switching) forwarding is a technique routers use to efficiently direct data packets through a network. Unlike traditional IP routing, MPLS forwarding employs labels to identify and forward packets along pre-determined paths quickly. By using labels, MPLS forwarding eliminates the need for complex IP lookups, resulting in faster and more streamlined data transmission.

Enhanced Performance: MPLS forwarding improves network performance by reducing latency and packet loss. Labels enable routers to make forwarding decisions more swiftly, resulting in lower transmission delays and increased overall efficiency.

Traffic Engineering: MPLS forwarding allows network administrators to engineer traffic paths based on specific requirements. By defining explicit paths for different types of traffic, MPLS enables better control over bandwidth utilization, ensuring that critical applications receive the necessary resources.

Quality of Service (QoS): MPLS forwarding enables the implementation of QoS policies, prioritizing certain types of traffic over others. This ensures that higher-priority applications, such as voice or video, receive the necessary bandwidth and experience minimal latency.

Understanding the Basics of LDP

LDP, or Label Distribution Protocol, is a signaling protocol that operates at the network layer. Its primary function is establishing and maintaining Label Switched Paths (LSPs) in a Multiprotocol Label Switching (MPLS) network. LDP enables routers to forward traffic along predetermined paths by assigning labels to network packets.

LDP Operation and Label Distribution

To comprehend LDP fully, it’s essential to understand how it operates and distributes labels. LDP uses a discovery mechanism to identify neighboring routers and establish peer relationships. Once peers are established, the protocol exchanges label mapping information, allowing routers to build forwarding tables and determine the appropriate paths for incoming packets.

Advanced Topic

Understanding BGP Next Hop Tracking:

BGP Next Hop Tracking is a mechanism that allows a router to track the reachability of the next hop IP address in the routing table. Essentially, it enables routers to dynamically adjust their routing decisions based on the availability of the next hop. By monitoring the reachability of the next hop, BGP Next Hop Tracking enhances network stability and enables faster convergence during link failures or network changes.

Implementing BGP Next Hop Tracking requires proper configuration on BGP-enabled routers. The specific steps may vary depending on the network equipment and software being used. Generally, it involves enabling the Next Hop Tracking feature and specifying the desired parameters, such as the interval for tracking next hop reachability and the action to be taken upon reachability changes.

BGP Next Hop Tracking is useful in various network architectures. It is commonly used in multi-homed networks, where multiple ISPs are connected to a single network. By tracking the reachability of the next hops, routers can intelligently select the best path for sending traffic and avoid blackholing or suboptimal routing. Additionally, BGP Next Hop Tracking is beneficial when network policies require specific routing decisions based on next-hop reachability.

Understanding BGP Route Reflection

BGP route reflection is a technique to reduce the number of full-mesh peerings required in a BGP network. Network administrators can simplify their network topology and improve scalability by introducing route reflectors. Route reflectors act as centralized points for route distribution, allowing BGP speakers to establish fewer connections while still effectively exchanging routing information.

In a route reflection setup, route reflectors receive BGP updates from their clients and reflect those updates to other clients. The reflection process involves modifying the BGP attributes to preserve the path information while avoiding routing loops. This enables efficient propagation of routing information across the network while reducing the computational overhead associated with maintaining a full mesh of BGP peers.

Related: Before you proceed, you may find the following posts helpful:

Highlights: Forwarding Routing Protocols

Switching and Routing

Before we get into the technical details of which protocol routers use to forward messages, let us address the basics. We know we have Layer 2 switches that create Ethernet LANs. So, all endpoints physically connect to a Layer 2 switch. And if you are on a single LAN with one large VLAN, you are prepared with this setup as switches work out of the box, causing conclusions based on Layer 2 MAC addresses. However, what if you want to send data from your network to another, across the Internet, or a different set of VLANs in different IP subnets?

In this case, we need a Layer 3 router and an IP routing process with an IP forwarding algorithm. So, do you want to know which protocol routers forward messages? The Layer 3 router uses the information in the IP header to determine whether and where to forward each received packet and which network interface to send the packet to.

Examples: Forwarding Routing Protocols

Routing Information Protocol (RIP)

One of the most commonly used forwarding routing protocols is the Routing Information Protocol (RIP). RIP is a distance-vector protocol that uses a metric, typically hop count, to determine the best path for data packets. It exchanges routing information with neighboring routers and updates its routing table accordingly. RIP is suitable for small to medium-sized networks due to its simplicity and ease of configuration.

Open Shortest Path First (OSPF)

Another widely used forwarding routing protocol is the Open Shortest Path First (OSPF) protocol. OSPF is a link-state protocol that calculates the shortest path to a destination based on various factors, such as bandwidth, delay, reliability, and cost. It advertises link-state information to neighboring routers, allowing them to build a complete network topology. OSPF is commonly implemented in large-scale networks due to its scalability and advanced features.

Border Gateway Protocol (BGP)

Border Gateway Protocol (BGP) is a forwarding routing protocol commonly used in internet service provider (ISP) networks. BGP is an exterior gateway protocol that facilitates the exchange of routing information between different autonomous systems (ASes). It enables ISPs to select the best path for data packets based on various policies, such as path length, network congestion, and customer preferences. BGP is crucial for maintaining a stable and efficient internet routing infrastructure.

Guide: OSPF

In the following lab guide, we address OSPF. OSPF, developed by the Internet Engineering Task Force (IETF), is an interior gateway protocol (IGP) used for routing within autonomous systems (AS). A link-state routing protocol uses the Shortest Path First (SPF) algorithm to determine the best path for forwarding data packets. OSPF is widely adopted due to its scalability, fast convergence, and support for multiple network types.

Note:

Notice that we have two OSPF neighbors. We use the default broadcast network type and have an OSPF status of FULL/DR. I have changed the OSPF cost on the link Gi1 so that we can perform traffic engineering. Now that the links have the exact OSPF costs, a total metric of 4, we can perform ECMP. You can also bond links; we combine two links for additional bandwidth.

Example: OSPF Routed Core

With a leaf and spine, we can have a routed core. So, we gain the benefits of running a routing protocol, such as OSPF, all the way down to the access layer. This has many advantages, such as full use of links. The guide below has three routers: two leaves and two spines. OSPF is the routing protocol with Area 0; we are not running STP.

Therefore, we can have Layer 3 routing for both spines to reach the destinations on Leaf B. I have a loopback configured on Leaf B of 1.1.1.1. Each leaf has an OSPF neighbor relationship to each spine with an OSPF network type of Broadcast. Notice the command: Show IP route 1.1.1.1 on Leaf A.

Note:

We initially only had one path via Spine B, i.e., the shortest path based on OSPF cost. Once I made the OSPF costs the same for the entire path (Cost of 4, routing metric of 4 ), we installed 2 paths in the routing table and can now rely on the fast convergence of OSPF for link failure detection and recovery.

We will expand this with one of the following lab guides in this blog with VXLAN and create a layer 2 overlay. Remember that ACI does not have OSPF and uses IS-IS; it also has a particular configuration for VXLAN, and much of the CLI complexity is abstracted. However, the focus of these lab guides is on illustration and learning.

The process of routing and network stretch

Routing is selecting a path for traffic in a network or between or among multiple networks. Routing is performed for various networks, including the Internet, circuit-switched, and packet-switched networks. The routing process usually directs forwarding based on routing tables, which maintain a record of the routes to various network destinations. Thus, constructing routing tables in the router’s memory is crucial for efficient routing.

Routing is typically based on the shortest path algorithm, which finds the shortest path from source to destination in a network. The shortest path algorithm can be implemented using various techniques, such as Dijkstra’s and Bellman-Ford’s algorithms. In addition, routing can also be based on other criteria, such as least cost, lowest delay, or highest reliability.

Routing Tables:

Routing protocols are used to maintain router routing tables. These protocols enable the routers to exchange information about the network topology, such as which nodes are connected, and then determine the best routes. The most common routing protocols are the Open Shortest Path First (OSPF) and the Routing Information Protocol (RIP).

Routing also ensures that data sent over the Internet reaches its destination. To do this, routers use the Internet Protocol (IP) to forward packets between networks. They examine the packet’s IP header and use this information to determine the best route for the packet.

Diagram: The routing process. The source is Baeldung.

Guide: EIGRP Configuration

EIGRP stands for Enhanced Interior Gateway Routing Protocol and is a routing protocol created by Cisco. Initially, it was only available on Cisco hardware, but for a few years, it’s now an open standard. EIGRP is called a hybrid or advanced distance vector protocol, and most of the rules that apply to RIP also apply here:

Split Horizon
Route Poisoning
Poison Reverse

EIGRP routers will send hello packets to other routers like OSPF; if you send and receive them, you will become neighbors. EIGRP neighbors will exchange routing information, which will be saved in the topology table

Configuring EIGRP is similar to RIP. The “1” is the AS number, which must be the same on all routers! We require the no auto-summary command because, by default, EIGRP behaves classfully, and we want it to be classless.

Next, let’s have a look at the routing table below. The first thing you might notice is that you see a “D” for the EIGRP entries. You see a “D” and not an “E” because the last one has already been taken for EGP, an old routing protocol we no longer use. “D” stands for “dual,” which is the mechanism behind EIGRP. The loopback 4.4.4.0 is connected to R4, and R1 has two ways to reach this network. This is because all links are Gigabit Ethernet, and I have not changed any metrics.

Routing vs Forwarding

Often, routing is confused with forwarding, but routing is a different process. When routing data, routers move data between devices. During data forwarding, a device collects data from one device and sends it to another. Let’s take a closer look at the forwarding process.

The forwarding process involves collecting data from one device and sending it to another. Data is not moved from one device to another in this process. In contrast to routing, forwarding performs some actions and forwards packets to intermediate routers. It does not determine the path. We only forward the packets to another attached network in the forwarding process.

The network layer performs both routing and forwarding. A forwarding device collects data and sends it to another. Hubs, routers, and switches are some of the most popular forwarding devices.

Guide: IS-IS Routing Protocol

In the following sample, we have an IS-IS network.

The ISIS routing protocol is a link-state routing protocol that operates at the OSI (Open Systems Interconnection) layer 2. It was initially developed for large-scale networks such as the Internet, where scalability, stability, and efficient routing are paramount.

Note:

Below, we have four routers. R1 and R2 are in area 12, and R3 and R4 are in area 34. R1 and R3 are intra-area routers so they will be configured as level 1 routers. R2 and R4 form the backbone so these routers will be configured as levels 1-2.

Key Features of ISIS Routing Protocol

Hierarchical Design: ISIS employs a hierarchical design, dividing the network into areas to simplify routing and improve scalability. Each region has a designated router, the Intermediate System (IS), which is responsible for exchanging routing information with other ISes.

Link-State Database: ISIS maintains a link-state database that contains information about the network topology and the state of individual links. This database calculates the shortest path to a destination and ensures efficient routing.

Dynamic Updates: ISIS uses a dynamic routing algorithm to exchange routing information between ISes. It continuously updates the link-state database based on network changes, ensuring the routing information is always current.

Support for Multiple Routing Protocols: ISIS is interoperable with protocols such as OSPF (Open Shortest Path First) and BGP (Border Gateway Protocol). This flexibility allows networks to integrate ISIS with existing routing infrastructures seamlessly.

Packet-switching Networks

The Internet is a packet-switching network that enables its attached devices, such as your personal computer ( PC ), to exchange information with other devices. Information exchange could take many different forms. From a user level, it could be checking your bank balance with Internet banking, buying a book on an Amazon website, watching a movie online, or downloading your favorite song.

Hypertext Transfer Protocol ( HTTP ) accounts for most Internet traffic and is the protocol behind the World Wide Web ( WWW ). However, for these upper-layer protocols ( HTTP ) to work efficiently and offer a satisfactory user experience, elements lower in the Open Systems Interconnection ( OSI ) communication module must be fine-tuned and operational for data transfers.

Diagram: Packet Switching Networks. Source is GeeksforGeeks.

Forwarding Protocols

Which protocol is used by routers to forward messages?

The two transport protocols

The TCP/IP protocol suite supports two transport protocols ( Layer 4 ): Transmission Control Protocol (TCP ) and User Datagram Protocol ( UDP ). TCP reliably provides a host-to-host communication service, while UDP provides host-to-host communication in an unreliable fashion.

As a result, TCP offers many services better suited for applications requiring certain service guarantees and error correction and detection, such as Border Gateway Protocol, which operates on Port 179. On the other hand, UDP offers fewer services and is helpful for situations where packet loss is less sensitive, but time delays are more problematic.

This information is traversed across the Internet backbone via the Network ( Layer 3 ) and Data Link layer ( Layer 2 ). It is encoded in long strings of bits called packets. Packets describe a chunk of data going from the IP ( Internet Protocol ) layer to the network interface ( Data Link Layer ).

Guide: BGP Next hop tracking

BGP Next Hop Tracking is a feature within the BGP routing protocol that allows routers to track the reachability of next-hop IP addresses. By monitoring the availability of next-hop routers, network administrators can make informed decisions regarding traffic routing and ensure efficient packet transmission.

BGP Next Hop Tracking offers several advantages in terms of network resilience. Firstly, it enables the identification and avoidance of black holes or suboptimal routing paths by detecting unreachable next-hop IP addresses. This ensures traffic is efficiently routed along viable paths, minimizing latency and potential packet loss. Additionally, BGP Next Hop Tracking facilitates faster convergence during network failures by swiftly redirecting traffic to alternate paths, reducing the impact of network disruptions.

The Packet and a Datagram

A packet is not the same as a datagram and can be either an IP datagram or a fragment of an IP datagram. Note: The terminology “packet” refers to the Ethernet payload, which consists of the IP header and the user data. The terminology frame refers to the data link headers and the payload.

As these packets travel through the Internet from their source ( your personal computer ) to their destination ( Amazon website ), certain decisions are made by each device the packet traverses. These are known as routing decisions and determine if the packet should go this way or that way.

The devices making these decisions are called routers. Different routers act at different network points, such as over the WAN with SD-WAN routers: SD WAN tutorial.

IP Packet versus IP Datagram
The diagram shows the different definitions of an IP packet compared to an IP datagram. It also shows how an IP datagram is fragmented into two IP packets, with the second IP packet being the second part of the first IP packet.

Diagram: IP packet vs Datagram. Source is crnetpacket

Routing Tables and Routing Protocols

These devices have a routing table that tells them how and where to forward the packets. The routing table is populated by a dynamic or static process called a routing protocol. A static routing protocol is specific to that device, manually configured, and is not automatically populated to other routers.

A dynamic process runs distributed algorithms that the routers run among themselves to make the correct routing decision.

An example of a dynamic routing protocol is OSPF, and a static routing protocol would be a static route. A router’s routing protocol may be Distance Vector Algorithms or Link-State Algorithms. Distance Vector Algorithms are more straightforward and usually try to find paths with a simple metric, such as the number of router hops ( devices ) to the destination.

On the WAN side, we have Border Gateway Protocol (BGP) and the use case of BGP SDN. We are enabling WAN virtualization and SDN traffic optimizations.

Guide: EIGRP

In the following, we have an EIGRP network that consists of two routers.

Note:

Efficient Exchange of Routing Information

One of EIGRP’s strengths is its ability to exchange routing information with neighboring routers. Using Hello packets and Update packets, EIGRP establishes and maintains neighbor relationships. This dynamic exchange ensures that routers are constantly updated with the latest network topology information, facilitating efficient route computation and decision-making.

For neighbor discovery and recovery, EIGRP neighbors send hello packets. If you send and receive hello packets, EIGRP will form a neighbor relationship with another router. If you receive hello packets from the other side, EIGRP will assume the other router is still present. When you no longer receive them, you’ll lose the neighbor relationship called adjacency, and EIGRP might have to look for another route.

EIGRP uses RTP (Reliable Transport Protocol) to deliver packets between neighbors in a reliable and orderly manner. There are two ways to send packets, multicast and unicast, and not all packets are sent reliably to keep things efficient. We need acknowledgment from the other side to ensure our packets are reliable.

Analysis:

Populating the Topology Table

EIGRP populates its topology table by exchanging Hello and Update packets with neighboring routers. These packets carry information about the network’s topology, such as feasible successors, advertised distances, and reported distances. As EIGRP receives these updates, its topology table will be updated accordingly.

Computing the Best Paths

Once the topology table is populated, EIGRP utilizes the DUAL algorithm to determine the best paths to reach destination networks. The algorithm considers bandwidth, delay, reliability, and load to calculate each route’s composite metric, the metric value. This metric value aids in selecting the optimal path for packet forwarding.

Maintaining and Updating the Topology Table

The EIGRP topology table is a dynamic entity that undergoes constant updates. EIGRP ensures that the topology table is kept current as changes occur in the network. When a link or router fails, EIGRP recalculates paths based on the remaining available routes and updates the topology table accordingly.

Routing convergence: Determine the Best Path

A router runs its algorithm and determines the best path to a particular destination; the router then notifies all of the neighboring routers of its current path; concurrently, the router’s neighbors also inform the router of their best paths. All of this occurs in a process known as routing convergence.

Rouitng Convergence	Forwarding in Networking
Detect
Describe
Switch
Find

After seeing all the other best paths from its neighboring devices, the router may notice a better path through one of its neighbors. If so, the router updates its routing table with better paths. A link-state algorithm employs a replicated database approach compared to a Distance Vector Algorithm ( distributed calculation ).

Each router contributes to database pieces; every device adds an element to create a complete network map. However, instead of advertising a list of distances to each known destination, the router advertises the states of its local links ( interfaces ).

Link state advertisements

These link-state advertisements are then advertised to the other routers; all these messages combine to complete a network database synchronized between each router at regular intervals.

Essentially, link-state protocols must flood information about the topology to every device in the network, and the distance ( path ) vector protocols must process the topology change information at every hop through the network.

A final note on forwarding protocols: Forwarding routing protocols

Routing protocols continually reevaluate their contents, and the process of finding new information after a change in the network is called convergence. A network deemed to be highly available must have not only a redundant physical topology but also fast convergence so that service degradation or interruption is avoided. Convergence should be designed efficiently at Layer 2 and Layer 3 levels.

Fast convergence of Layer 2 environments is designed with the Spanning Tree Protocol ( STP ) enhancements, notably PVST+. In L3 environments, we prefer routing protocols that can quickly find new information ( next hops ), with protocols having a short convergence.

You might conclude from the descriptions of both link-state and distance-vector protocols that link-state algorithms will always converge more quickly than distance or path-vector protocols. However, this isn’t the case; both converge exceptionally promptly if the underlying network has been designed and optimized for operation.

Closing Points: Forwarding Routing Protocols

Forwarding routing protocols play a crucial role in efficiently transmitting data across networks. This blog post delved into forwarding routing protocols, exploring their significance, functionality, and types. By the end, you will clearly understand how these protocols enable seamless communication between devices on a network.

Forwarding routing protocols have several key benefits that make them essential in network communication:

1. Scalability: Forwarding routing protocols enable networks to expand and accommodate a growing number of devices. These protocols dynamically adapt to changes in network topology, allowing for the seamless integration of new devices and routes.

2. Redundancy: Forwarding routing protocols continuously exchange routing information to ensure alternative paths are available in case of link failures. This redundancy enhances network reliability and minimizes downtime.

3. Load Balancing: Forwarding routing protocols distribute network traffic across multiple paths, optimizing network performance and preventing congestion. This feature allows for efficient utilization of network resources.

Types of Forwarding Routing Protocols:

Various forwarding routing protocols are designed to cater to specific network requirements. Let’s explore some of the most commonly used types:

1. Distance Vector Protocols:

Distance vector protocols, such as the Routing Information Protocol (RIP), use a simple approach to determining the best path. Routers exchange their routing tables, which contain information about the distance and direction of various network destinations. RIP, for example, evaluates paths using hop count as a metric.

2. Link State Protocols:

Link state protocols, such as Open Shortest Path First (OSPF), build a detailed database of the network’s topology. Routers share information about their directly connected links, allowing each router to construct a complete network view. This comprehensive knowledge enables OSPF to calculate the shortest path to each destination.

3. Hybrid Protocols:

Hybrid protocols, like Enhanced Interior Gateway Routing Protocol (EIGRP), combine elements of both distance vector and link state protocols. These protocols balance simplicity and efficiency, utilizing fast convergence and load-balancing features to optimize network performance.

Forwarding routing protocols are essential for ensuring reliable and efficient data transmission in computer networks. By determining the optimal paths for data packets, these protocols contribute to the overall performance and stability of the network. Understanding different forwarding routing protocols, such as RIP, OSPF, and BGP, is crucial for network administrators and engineers to design and manage robust networks.

Forwarding protocols are vital in modern networking, enabling efficient data routing and seamless network communication. Understanding these protocols’ different types, benefits, and challenges is crucial for network administrators and engineers. Organizations can confidently navigate the digital highway by implementing best practices and staying abreast of advancements in forwarding routing protocols.

Summary: Forwarding Routing Protocols

In the vast landscape of computer networks, efficient data transmission is critical. Forwarding routing protocols play a crucial role in ensuring that data packets are delivered accurately and swiftly. In this blog post, we explored the world of forwarding routing protocols, their types, and their significance in modern networking.

Understanding Forwarding Routing Protocols

Forwarding routing protocols are algorithms routers use to determine the best path for data packets to traverse through a network. They enable routers to make informed decisions based on various factors such as network topology, cost metrics, and congestion levels. These protocols optimize network performance and ensure reliable data transmission by efficiently forwarding packets.

Types of Forwarding Routing Protocols

There are several forwarding routing protocols, each with its characteristics and use cases. Let’s explore a few prominent ones:

Distance Vector Protocols

Distance Vector protocols, such as Routing Information Protocol (RIP), share routing information with neighboring routers. They exchange routing tables periodically, making routing decisions based on the number of hops to reach a destination. While simple to implement, distance vector protocols may suffer from slow convergence and limited scalability.

Link State Protocols

Link State protocols, like Open Shortest Path First (OSPF), take a different approach. Routers in a link state network maintain detailed information about the network’s topology. Routers build a comprehensive network view by flooding link state advertisements and calculating the shortest path to each destination. Link state protocols offer faster convergence and better scalability but require more computational resources.

Hybrid Protocols

Hybrid protocols, such as Enhanced Interior Gateway Routing Protocol (EIGRP), combine the advantages of both distance vector and link state protocols. They offer the simplicity of distance vector protocols while providing faster convergence and better scalability. Hybrid protocols are widely used in enterprise networks.

Significance of Forwarding Routing Protocols

Forwarding routing protocols are crucial for efficient network operations. They bring several benefits to the table:

Optimal Path Selection

By analyzing network metrics and topology, forwarding routing protocols enable routers to choose the most efficient path for packet forwarding. This results in reduced latency, improved network reliability, and better overall performance.

Load Balancing

Many forwarding routing protocols support load balancing, distributing traffic across multiple paths. This helps prevent congestion on certain links and ensures efficient resource utilization throughout the network.

Fault Tolerance

Forwarding routing protocols often incorporate mechanisms to handle link failures and reroute traffic dynamically. In case of link failures, routers can quickly adapt and find alternative paths, minimizing downtime and maintaining network connectivity.

Conclusion:

In conclusion, forwarding routing protocols are the backbone of modern computer networks. They provide the intelligence needed for routers to make informed decisions, ensuring efficient packet forwarding and optimal network performance. By understanding the different types and significance of forwarding routing protocols, network administrators can design robust and scalable networks that meet the demands of today’s digital world.

Routing Convergence

Highlights: Routing Convergence

Understanding Convergence

Convergence Process

Mechanisms for Achieving Routing Convergence:

Strategies for Achieving Optimal Routing Convergence

BGP Next Hop Tracking

**Benefits of Efficient Routing Convergence:**

Example Technology: BFD

Convergence Time

Topological information

Understanding UDLD

Enhancing Routing Convergence

**Factors Influencing Routing Convergence**

Control and data plane

Monitoring and Troubleshooting Routing Convergence

Routing Convergence

Convergence Time Definition.

**IP routing**

Knowledge Check: Bidirectional Forwarding Detection

Convergence Routing and Network Convergence Time

Example: OSPF

OSPF: Incremental SPF

Forwarding Paradigms

**Finding paths across the network**

Convergence Routing

Convergence routing: EIGPR

Lab Guide: EIGRP LFA FRR

TCP Congestion control

Adding Resilience

Routing Convergence and Routing Protocol Algorithms

**Stage 1: Failure Detection**

Layer 1 failure detection

Layer 2 failure detection

Layer 3 failure detection

**Stage 2: Routing convergence and failure propagation**

**Stage 3: Topology/Routing calculation**

**Stage 4: Update the routing and forwarding table ( RIB & FIB)**

Closing Points: Routing Convergence

Summary: Routing Convergence

Forwarding Routing Protocols

Highlights: Forwarding Routing Protocols

Routing Protocols

Protocol Algorithms:

Example: Understanding RIP

Example: OSPFv3

Starting points: Networking Protocols

Forwarding Methods

The Role of Switching

IP Routing

Rules and Algorithms

Example: EIGRP DUAL

Example: MP-BGP with IPv6 Prefixes

**Common Forwarding Routing Protocols**

Understanding MPLS Forwarding

Understanding the Basics of LDP

Advanced Topic

Understanding BGP Next Hop Tracking:

Understanding BGP Route Reflection

Highlights: Forwarding Routing Protocols

Switching and Routing

Examples: Forwarding Routing Protocols

Guide: OSPF

Example: OSPF Routed Core

The process of routing and network stretch

Routing Tables:

Guide: EIGRP Configuration

Routing vs Forwarding

Guide: IS-IS Routing Protocol

**Key Features of ISIS Routing Protocol**

Packet-switching Networks

Guide: BGP Next hop tracking

The Packet and a Datagram

Routing Tables and Routing Protocols

Guide: EIGRP

Closing Points: Forwarding Routing Protocols

Summary: Forwarding Routing Protocols

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Benefits of Efficient Routing Convergence:

Factors Influencing Routing Convergence

IP routing

Finding paths across the network

Stage 1: Failure Detection

Stage 2: Routing convergence and failure propagation

Stage 3: Topology/Routing calculation

Stage 4: Update the routing and forwarding table ( RIB & FIB)

Common Forwarding Routing Protocols

Key Features of ISIS Routing Protocol