EIGRP Routing Design Question

Unanswered Question
Apr 1st, 2009

hello all.


i inherited a large frame-relay hub and spoke network with multiple hubs and using only static routing. there are about 10 hubs, and each remote router is connected to one of these hubs, plus a backup hub (the same for every router). we would like to implement a dynamic routing protocol, and i have experience with OSPF and EIGRP. we've decided on EIGRP, but here's the issue and hence my question: the WAN addresses *within* each hub are unique, but there is overlap from one hub to another. for instance, 10.1.1.1 is a WAN interface on Hub1, Hub2, Hub3, etc. Is this a problem for EIGRP? We don't really care about learning the WAN routes, just the loopbacks on the remote routers and some static routes, both of which *are* unique, unlike the WAN addresses.


i hope i made this clear. i understand the IP overlap is not ideal, but getting those changed is an even bigger project than implementing a routing protocol, so if i can avoid it, i will. i believe EIGRP should work, but i know

OSPF won't, because it won't be able to build an accurate picture of the network with all that overlap.


thanks for your time!


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lamav Wed, 04/01/2009 - 08:20

" for instance, 10.1.1.1 is a WAN interface on Hub1, Hub2, Hub3"


Huh? You lost me...can you elaborate further on that?


From my experience, the most efficient way to implement eigrp on a frame relay hub and spoke environment is to create point-to-point subinterfaces on the hub and spoke, with /30 subnets on the WAN links.


HTH


Victor

johndennison Wed, 04/01/2009 - 08:34

sorry, yes, that is what we have done. the problem is that 10.1.1.0/30 is used for a WAN subinterface on Hub1, Hub2, and Hub3, etc.

lamav Wed, 04/01/2009 - 08:36

Im still not understanding it.


Are you saying you have duplicate addresses on your network?? How can you assign the same subnet to 3 different serial interfaces on 3 different routers?


What am I missing?

Giuseppe Larosa Wed, 04/01/2009 - 08:44

Hello John,

OSPF is out of discussion.


EIGRP can support this but:


be aware that EIGRP external routes have a field that contain the EIGRP router-id.


So you need to verify that the overlapping ip address is not chosen by any router as eigrp router-id.


sh ip eigrp topology should report the router-id.


in case you can force the EIGRP router-id with eigrp router-id command


see


http://www.cisco.com/en/US/docs/ios/12_2/iproute/command/reference/1rfeigrp.html#wp1031884


Readdressing can be performed on a link by link basis provided that you access the remote router from the backup hub (that I hope it is fine) make your changes on the primary link and verify connectivity.


I would think of readdressing you can do it gradually over time.


Hope to help

Giuseppe


lamav Wed, 04/01/2009 - 08:58

Alrighty then....


Giuseppe is right, of course. You need to re-address your network. Honestly, I have never seen such a cockamamie set up. Why on earth would anyone deliberately assign duplicate IP addresses on their network? What could that possibly buy them?

johndennison Wed, 04/01/2009 - 09:09

Giuseppe, you've confirmed my thoughts, thank you.


FYI, we would configure the loopback on each router to be the EIGRP router-id.

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