IP Forwarding Example

Forwarding Routing Protocols


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


Highlights: Forwarding Routing Protocols

  • What is IP Routing

What is IP routing? Let’s start to understand which protocol routers use 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.

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


  • A key point: Video on multi-path TCP with IP video routing.

In this IP video routing, we will address multi-path TCP, a new approach to how TCP operates, and is commonly used in new data center topologies such as the spine leaf architecture. TCP’s main drawback is that it’s a single path per connection protocol.

A single path means once the stream is placed on a path ( end points of the connection), it can not be moved to another path even though multiple paths may exist between peers.



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 on 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.


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.


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 on 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.


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.

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 area 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 ).


  • A key point: Video on 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 a set of predefined rules used by the routers that interconnect your network to maintain the communication between the source and the destination. These routing protocols help to find the routes between two nodes on the computer network.



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 shows how an IP datagram is fragmented into two IP packets. So, the second IP packet is 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 on EIGRP

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


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.


 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


    • 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 factors like bandwidth, delay, reliability, and load to calculate each route’s composite metric, known as 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






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 a piece 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 an entire network database synchronized between each router through 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 means not only having a redundant physical topology but also having 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 quickly if the underlying network has been designed and optimized for operation. 


Recap on 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.

In conclusion, forwarding routing protocols are a fundamental aspect of computer networks. They enable the efficient transmission of data packets by determining the most optimal paths. Examples of forwarding routing protocols include RIP, OSPF, and BGP, each with its unique characteristics and use cases. By understanding and implementing these protocols, network professionals can ensure the reliability and performance of their networks.


which protocol is used by routers to forward messages?

Matt Conran
Latest posts by Matt Conran (see all)

Comments are closed.