Network engineers are frequently asked questions like: "My network currently runs Enhanced Interior Gateway Routing Protocol (EIGRP). Would I be better off if I switched to Open Shortest Path First (OSPF)?"
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You can replace the two protocols mentioned in this sentence with any pair of protocols among the advanced interior gateway protocols (OSPF, IS-IS and EIGRP), and you have described a question that routing protocol engineers are asked probably thousands of times a year. Of course, convergence is always faster on the other side of the autonomous system boundary, so to speak, so it is always tempting to jump to another protocol as soon as a problem crops up with the one you are running.
How do you answer this question in real life? You could try the standard, "It depends," but does this really answer the question? The tactic in the Routing Protocols Escalation Team was to ask them questions until they went away, but none of these answers really helps the network operator or designer really answer the question, "How do you decide which protocol is the best?" Three questions are embedded within this question, really, and it is easier to think about them independently:
- Is one protocol, in absolute terms, "better" than all the other protocols, in all situations?
- If the answer to this first question is "No," does each routing protocol exhibit some set of characteristics that indicate it would fit some situations (specifically, network topologies) better than others?
- After you have laid out the basics, what is the tradeoff in living with what you currently have versus switching to another routing protocol? What factors do you need to consider when doing the cost/benefit analysis involved in switching from one routing protocol to another?
This chapter excerpt from Optimal Routing Design by Russ White, Alvaro Retana and Don Slice takes you through each of these three questions. This might be the first and last time that you hear a network engineer actually answer the question, "Which routing protocol should I use?" so get ready for a whirlwind tour through the world of routing.
Read the rest of Which routing protocol? in this PDF.