I ran into Tony Hain at NANOG last week, and he told me he had gotten pounded with questions about IPv4 address exhaustion after my post discussing his and Geoff Huston’s studies. His response to them was something to the effect of, “Jeff just repeated what I’ve been saying for years. Why are you asking now?” My response to Tony was “Jeez, I didn’t think anyone was reading my posts!” I think I’ve gotten more comments since I stopped posting than I did when the blog was active. Anyway, I decided to start it up again.
And a good place to start is with a request from Murilo, in response to the last post:
I would like to ask you if you could speak a bit about EIGRP vs OSPF. Both are IGP protocols and if you have a network only with Cisco routers what is the best option?
With apologies to my friends at Cisco, I have to say that I’ve never recommended EIGRP to any of my clients. I’ve worked with many who have already made up their mind in favor of EIGRP and I’ve acquiesced to their wishes, but if I’m asked I adamantly recommend OSPF.
For years I’ve referred to EIGRP as a consultant’s best friend. Its easy to configure, doesn’t require you to think much about your network topology, and works very well in networks up to a certain size. Just slap another router in the network as needed, turn on EIGRP, and you’re done. But then when your network grows large enough to need some scaling limits, forcing you to finally think about your topology, untangling EIGRP can be daunting. That’s when many operators call a consultant like me, who is happy to come in and implement an EIGRP to OSPF migration project for lots and lots of money. So in that mercenary way I’m quite fond of the protocol.
The primary scaling limitation with EIGRP is that it doesn’t have a capability for setting internal boundaries, important for controlling prefix summarization and database sizes, the way OSPF areas do. You can artificially do this by using multiple EIGRP processes, but why use a kludge to accomplish something OSPF does as an integral part of the protocol?
The above is not to say areas are always a good thing, either. An interesting phenomenon I’ve observed over the years is that while EIGRP networks tend to get out of control because they remain a single, flat domain as the network expands, many OSPF designers go to the other extreme and overuse areas. I’ve seen networks of 50 or so OSPF routers, which would operate just fine as one big area, needlessly divided into more than a dozen areas.
Where EIGRP scaling problems usually become evident is with stuck-in-active (SIA)
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