Introduction to Routing Protocols

Today we’ll review routing protocols and list the most popular ones. This is a good segway from my previous post where I covered IPv4 Subnetting. Before moving on to routing protocols, I want to clarify the difference between routing and forwarding.

Forwarding is an internal host function that works by transmitting a packet from an incoming interface to an outgoing interface with the goal of reaching a remote host or network. When making forwarding decisions hosts consult a local lookup table, called routing table. The routing table contains a list of destination networks along with the next hop or interface that should be used to reach a specific destination network. In essence, routing protocols build a model of the underlying network topology.

What’s a routing protocol?

A routing protocol is a process that enables network routers decide which routes will be used when making forwarding decisions. Generally a routing process must have the following information to build the entire routing table:

  • The destination address
  • Neighbor routers to reach remote networks
  • Available routes to reach remote networks
  • The best route to each remote network
  • How to verify and maintain routing information

Static vs. Dynamic Routing Protocols

There are two ways to configure network routers to forward traffic: static or dynamic routing. In a static routing configuration, the network administrator has to manually program the routing table of each individual router. This is done by configuring static routes via command line or graphic interface. Static routing is a very tedious process, prone to errors, and not scalable. It should not be used unless the network is very small (e.g. less than 10 routers).

A dynamic routing protocol automates the exchange process of routing information amongst routers. A dynamic routing protocol enables routers to update their routing tables without intervention from the network administrator. Dynamic routing is the way enterprise networks are configured and traffic flows from one network to another.

Routing Protocols: Internal vs External

A routing protocol defines how a group of routers exchange information to forward packets amongst them. There are two types of routing protocols: interior and exterior. Interior Gateway Protocols (IGP) are configured on routers managed by the same organization and that belong to the same Autonomous System. Exterior Gateway Protocols (EGP) instead are configured on routers belonging to different organizations, or Autonomous Systems.

These are the most common routing protocols:

OSPF (Open Shortest Path First) – Internal

OSPF is an open standard routing protocol that is widely used across multi-vendor networks. It’s a link state routing protocol so called because it establishes neighboring relationships with other routers and sends advertisements to nearby routers to communicate routing information.

IGRP (Interior Gateway Routing Protocol) – Internal

IGRP is a distance vector routing protocol that was developed by Cisco. This routing protocol is now obsolete, replaced by its next generation version. It was used across smaller to medium sized Cisco-based networks.

EIGRP (Enhanced Interior Gateway Routing Protocol)- Internal

This is the successor to IGRP and carries much of the same characteristics. Some of the main advantages include: faster convergence, support for classless networks, requires less bandwidth thanks to incremental updates, and supports neighbor tables.

RIPv2 (Routing Information Protocol V2) – Internal

RIP is an open standard distance vector protocol that was released after its version 1 and with support for classless routes. RIPv2 calculates routers based on hop count only, without taking into account other factors such as link speed, etc. RIP suffered from scalability issues as it doesn’t support routers with more than 15 hops, and has a high convergence time, due to the time that it takes to recalculate new routes. For these reasons, this protocol is obsolete and replaced with OSPF.

iBGP (Internal Border Gateway Protocol – Internal

iBGP is an open standard IGP that has lower bandwidth requirements for routing updates, supports up to 255 hops, and supports multi-path routing (for high availability). For this reason, BGP is called a path vector protocol. One downside of iBGP is that to function properly the participating routers need to be linked in a full mesh configuration. Generally iBGP deployments have very specific use cases (e.g. MPLS) or are used in conjunction with eBGP ones. iBGP is an interior gateway protocol, which means that routers exchanging iBGP updates must be part of the same Autonomous System (AS).

eBGP (External Border Gateway Protocol) – External

This is often referred to as just BGP and has superseded the legacy routing protocol called EGP (External Gateway Protocol), which was used before the Internet as we know became popular. eBGP is a path-vector routing protocol currently used by Internet routers to select the best path to reach third parties’ networks. Without eBGP, the Internet would not function and enable the interconnection of billions of devices.

Routing Protocols Comparison

The following list groups different routing protocols into categories.

  • Classless routing protocols: RIPv2, EIGRP, OSPF, IS-IS, BGP
  • Classfull routing protocols: RIP, IGRP
  • Interior gateway protocols: RIP, RIPv2, IGRP, EIGRP, OSPF, IS-IS, iBGP
  • Exterior gateway protocols: eBGP
  • Link state routing protocols: OSPF, IS-IS
  • Distance vector routing protocols: RIP, RIPv2, IGRP, EIGRP
  • Path vector routing protocols: BGP

NetBeez and IP Routing

Understanding the performance of an enterprise network’s IP routing is crucial for IT operations and network engineers. Monitoring changes in IP routes is a core part of the NetBeez solution – it creates end-to-end tests that verify connectivity and performance from point A to point B. Via NetBeez, it’s possible to verify how routes change, thanks to path analysis, full-mesh of ping tests, and scheduled iperf tests. If you are interested in learning more, schedule a product tour.


IP Routing is fundamental in our line of work. Understanding how packets are sent and received across internal and external networks is very important. Especially when transmissions are failing, you need to have a basic understanding of IP Routing to know where to start troubleshooting.

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