routing convergence

Routing Convergence

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).

Table of Contents

Highlights: Routing Convergence

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, as well as the routing protocols used. Convergence can happen relatively quickly in smaller networks, while more extensive networks may take longer to achieve convergence.

To optimize routing convergence, network administrators can employ various strategies. 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 changes in the network.

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., the moving of the 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 be dynamic to changes in the network, both of the LAN and for WAN.

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

  1. Implementing Network Security
  2. Dead Peer Detection
  3. IPsec Fault Tolerance
  4. WAN Virtualization
  5. Port 179



Convergence Routing


Key Routing Convergence Discussion Points:


  • Convergence time definitions.

  • IP Forwarding paradigms.

  • Path Selection.

  • The effects of TCP congestion controls.

  • Adding resilience.

  • Routing protocol convergence steps.

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.

Back to Basics: IP routing

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 two means: 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.

Dynamic Routing Protocols
Diagram: Dynamic Routing Protocols. Source Cisco Press.

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.

Lab Guide: OSPF

The following lab guide demonstrates OSPF and its ability to perform ECMP. ECMP is performed with the total metric (i.e., OSPF costs ) being the same end-to-end for two links. In the screenshot below, also notice the default broadcast and DR election network types.

Diagram: Leaf and Spine Routed.

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.

Example Convergence Time with OSPF
Diagram: Example Convergence Time with OSPF. Source INE.

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.

Video: IP Forwarding

The following whiteboard video discusses the role of IP forwarding in networking. In this video, we will discuss switches and VLANs and then move to the basics of IP forwarding. So, we have networks that are broken down into different VLANs. So, we will have a group of switches linked together via trunk ports that provide connectivity for VLANs across different physical distances. The routers are used to route between different subnets.

IP Forwarding
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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.

So when we are up and running, events that force or make the routing protocols react and perform a convergence routing state can happen to the topology. 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 to be 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.

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

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. But 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.

Video: TCP Congestion Control

The discrepancy and uneven bandwidth allocation for flow boil down to the natural behavior of how TCP reacts and interacts with insufficient packet buffers and the resulting packet drops. The behavior is known as the TCP/IP bandwidth capture effect.

The TCP/IP bandwidth capture effect does not affect the overall bandwidth but more individual Query Completion Times and Flow Completion Times (FCT) for applications. The QCT and FCT are prime metrics for measuring TCP-based application performance.

Tech Brief Video Series – Networking | TCP Congestion Control
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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 do, the more data in the routing databases, the longer 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 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 – loss of carrier 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.

network convergence time
Diagram: Network convergence time and Layer 1.

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 and is needed when there are intermediate Layer 2 hops over Layer links and when you have concerns over uni-direction failures on point-to-point physical links.

Diagram: Layer 3 failure detection

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 are enabling Bidirectional Forwarding Detection ( BFD ) as the Layer 3 detection mechanism is strongly recommended over aggressive protocol times and using 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 Layer 3 failure detection 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.

Routing convergence
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, for example, link-state advertisement delay / initial link-state advertisement throttle delay.

Unfortunately, no such timers exist for Distance Vector Protocols. 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, this is where link-state protocols use exponential backoff timers. These timers adjust the waiting time OSPF and ISIS 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 in 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.

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

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.

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.

Conclusion: Routing convergence is an indispensable component of network management. Network administrators can ensure efficient and reliable communication by employing appropriate routing protocols, fast convergence techniques, and redundancy mechanisms, enhancing network performance and user satisfaction.

IP Forwarding Example

Forwarding Routing Protocols

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.

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.

Table of Contents

Highlights: Forwarding Routing Protocols

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 are responsible for directing data packets from the source device to the correct destination device, ensuring reliable and timely delivery.

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.

Forwarding Protocols.

Key Forwarding Routing Protocols Design Discussion Points:


  • Introduction to forwarding routing protocols and what is involved.

  • Highlighting the details of the TCP/IP suite.

  • Technical details on the packet and the datagram. 

  • Scenario: Routing tables and forwarding.

  • Details on routing convergence and path selection.

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

  1. IP Forwarding
  2. Routing Convergence
  3. OpenFlow Protocol
  4. IPsec Fault Tolerance
  5. BGP SDN
  6. ICMPv6
  7. SDN Router
  8. Segment Routing
  9. Routing Control Platform
  10. Computer Networking

Back to Basics: 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 the process of IP routing with an IP forwarding algorithm. So, if you want to know which protocol routers use to 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

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.

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 topology of the network. OSPF is commonly implemented in large-scale networks due to its scalability and advanced features.

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.

Lab 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.

Forwarding Routing Protocols
Diagram: Leaf and Spine Routed.

Lab Guide: Displaying Routed Core.

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 benefits, 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.

Layer 3 routed core
Diagram: Layer 3 routed core

 

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 protocols are used to maintain the routing tables in routers. 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. Routers examine the IP header of the packet and use this information to determine the best route for the packet.

The routing process
Diagram: The routing process. The source is Baeldung.

Routing vs Forwarding

Often, routing is confused with forwarding, but routing is a different process. Routers move data between devices when routing data. 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. The path is not determined by it. 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.

Lab 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 that they will be configured as level 1 routers. R2 and R4 form the backbone, so that these routers will be configured as levels 1-2.

Routing Protocol
Diagram: Routing Protocol. ISIS.

♦ 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), 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.

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

2.4. 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, for example, your personal computer ( PC ), to exchange information with other devices. The 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 ) makes up most of the 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. 

Packet Switching Networks
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 that 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.

Port 179
Diagram: Port 179 with BGP peerings.

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 ).

Video: BGP in the Data Center

In this whiteboard session, we will address the basics of BGP. A network exists specifically to serve the connectivity requirements of applications, and these applications are to serve business needs. So, these applications must run on stable networks, and stable networks are built from stable routing protocols.

Routing protocols are predefined rules used by the routers that interconnect your network to maintain communication between the source and the destination. These protocols help to find routes between two nodes on the computer network.

BGP in the Data Center
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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.

IP packet vs Datagram
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 by what is known as 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.

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

Lab Guide: EIGRP

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

Note:

Efficient Exchange of Routing Information

One of the strengths of EIGRP lies in 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.

EIGRP

 For neighbor discovery and recovery, EIGRP neighbors send hello packets. EIGRP will form a neighbor relationship with another router if you send hello packets and receive them. 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.

EIGRP topology

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, it updates its topology table 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, it may be the case that the router notices that there is 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 ).

routing convergence
The well-known steps in routing convergence.

 

  • A key point: 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. 

Recap: 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: By continuously exchanging routing information, forwarding routing protocols ensure that 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 Routing Information Protocol (RIP), use a simple approach to determine the best path. Routers exchange their routing tables, which contain information about the distance and direction of various network destinations. RIP, for example, uses hop count as a metric to evaluate paths.

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.

Conclusion:

Forwarding protocols are vital in modern networking, enabling efficient data routing and ensuring seamless communication across networks. 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.