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WAN Routing Protocols

Routing protocols can be classified in many ways, including operational characteristics, such as their field of use and the number of redundant routes to each supported destination. Dynamic routing protocols are categorized as Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs). IGPs are used within an autonomous system, in other words, within a single controlling entity, whereas EGPs are used between autonomous systems.

Key Features of Cisco IOS Supported Interior Gateway Protocols

Distance-Vector Protocols:
Link-State Protocols:
Distance-Vector and Link-State Protocols:
Key Features of Cisco IOS Supported Exterior Gateway Protocols

Path-Vector Protocols:
Routing Protocols

Routing protocols have far-reaching effects on the performance and reliability of your network, but sorting out the differences between them can be a challenge. Choose the best one for your network by taking a look at this detailed comparison.
Many of us work on networks we didn’t have the luxury of designing—with routing protocols we weren’t given the chance to choose. But you may be starting a new network, or you may have the opportunity to redesign your existing one. If you're tasked with selecting the best routing protocol for your network, which will you choose?
While there is often a “right tool for the job,” all routing protocols have their strengths and weaknesses, and I don’t believe that there is a clear-cut best routing protocol that is the right tool for every network. Thus, to help you select the most appropriate routing protocol for your network, I am going to examine the pros and cons of the best-known routing protocols and offer some guidance on when it makes sense to use one or the other.

RIP V2—Routing Information Protocol

Origin: Based on RFC 1388

Type of protocol: Distance vector, based on the Bellman-Ford distance vector algorithm

Metric: Hop count

Methodology: Selects routers with the lowest hop count; updates other routers by multicasting the entire routing table to all routers every 30 seconds

Ideal topology: Smaller networks that aren’t very dynamic, have fewer than 15 hops

Strengths: Weaknesses: IGRP—Interior Gateway Routing Protocol

Origin: Based only on Cisco’s implementation, not an Internet RFC

Type of protocol: Distance vector, based on the Bellman-Ford distance vector algorithm

Metric: Delay, bandwidth, reliability, and load

Methodology: Sends hello packets every five seconds to neighbors to see if the neighbor is still available; updates other routers by notifying them only when routes change

Ideal topology: Any network, small to very large; all routers must be from Cisco. Cannot subnet network beyond classful boundaries.

Strengths: Weaknesses: Definition - Convergence: The process that a routing protocol goes through to alert all routers on the network of the next available path when the primary path becomes unavailable.

EIGRP—Enhanced Interior Gateway Routing Protocol

Origin: Based only on Cisco’s implementation, not an Internet RFC

Type of protocol: Hybrid distance vector

Metric: Delay, bandwidth, reliability, and load, using the Diffusing Update Algorithm (DUAL)

Methodology: Sends hello packets every five seconds to neighbors (can interoperate with IGRP) to see if the neighbors are still available; updates other routers by notifying them only when routes change

Ideal topology: Any network, small to very large; all routers must be Cisco

Strengths: Weaknesses: OSPF V2—Open Shortest Path First
[Note that version 1 of OSPF was never implemented.]

Origin: Based on RFC 2328

Type of protocol: Link-state, runs the Dijkstra algorithm to calculate the shortest-path first (SPF) tree

Metric: Calculates the cost to traverse router links to get to the destination, taking the bandwidth of the links into account

Methodology: Develops adjacencies with its neighbors, periodically sending hello packets to neighbors, flooding changes to neighbors when a link’s status changes, and sending “paranoia updates” to neighbors every 30 minutes of all recent link state changes

Ideal topology: Any network, small to very large

Strengths: Weaknesses: Final word
Just to clarify this comparison, one way routing protocols are classified is according to how they are used. Interior routing protocols are used within a single domain on your interior network. Also called an Interior Gateway Protocol (IGP), this is the type of routing protocol you usually think of using for your internal network. The protocols we looked at in this article are all IGPs. Note that I omitted a few of the lesser-known interior routing protocols to keep the discussion reasonably short. These include IS-IS, NLSP, RTMP, and IPX RIP.

Another type of routing protocol is an exterior routing protocol, or Exterior Gateway Protocol (EGP). These protocols maintain routing information for networks that are external to your network. An EGP doesn’t know how to deliver data within your network, just how to deliver data outside your network. While a variety of IGPs are currently used, about the only EGP in use today is the Border Gateway Protocol (BGP). This is the routing protocol of the Internet.

From talking with administrators who manage a variety of networks, the consensus is that OSPF is becoming the most popular interior routing protocol today. I would recommend OSPF or EIGRP for any new network, based on their popularity, flexibility, and fast convergence. Of course, the choice is yours based on the requirements of your network. This article should help you get a better understanding of your options.

Additional references

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