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IBGP Configuration Question

Unanswered Question
Jan 21st, 2004
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I currently have three routers, we'll call them routers A, B, and C, where I'm routing EBGP to external providers, and where I have IBGP on each with belonging to the same peer group. Each of these three routers are c7500s.

Question: I'm bring up EBGP on another router. Will I need to route IBGP in the same peer group on this router? There are two intermediary routers between this new router where I'm turning up EBGP. So, the topology is:


Routers D, E, and F are all c7206s, with 128M ram.

I'll be bringing up up EBGP on router F.

I am not using route reflection.

Do I need to route IBGP on router F? Or can I just let the BGP on router F inject into the routing protocol (OSPF) ?

Thanks in advance.

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Harold Ritter Wed, 01/21/2004 - 11:40
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I presume you are not receiving full routes from the Internet if you say that you could redistribute BGP into OSPF on router F. It would still be a good idea for consistency purposes to run iBGP on all of the 6 routers. Otherwise you might end up blackholing the traffic on the routers not runnin iBGP.

Hope this helps,

bmisenor Wed, 01/21/2004 - 14:06
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Thanks for the reply. The plan is to receive full routes from ebgp on router F. I believe it is rational to run ibgp on router F, as well as D and E, though I seek the reasoning to justify doing this for others here. How would we inadvertently blackhole routes, though ?

Harold Ritter Wed, 01/21/2004 - 15:19
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If the border routers exchange Internet routes via iBGP with a IGP next hop going through router E and D and that these routers do not have a route for these specific routes, tha packets will be dropped.

n-saquib Thu, 01/22/2004 - 01:09
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I would suggest to disable sync on ebgp and run default routing on D and E. It will only save you to run iBGP on D and E but will not give you best path exit.

if you want to have best path exit than use iBGP with Route reflectors

Harold Ritter Thu, 01/22/2004 - 07:07
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The issue is that you can't efficiently use default routing on E and D when you have multiple exit point. This could also lead to blackholing the traffic.

n-saquib Thu, 01/22/2004 - 07:17
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but i would not like to suggest full routing on 72xx with 128MB RAM i have seen problems that router disable CEF due to memory problem. yes if you have easy way to get the RAM to 256 than could be best solution with Route reflectors

bmisenor Thu, 01/22/2004 - 07:41
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Thanks for the reply. I'm planning to upgrade the c7206 Router F to replace the NPE-300 with a NPE-G1. I am also concerned about memory utilization on the 7206 with just 128M ram. I am mainly concerned about scalability and functionality for IBGP between F and routers A, B, and C.

Harold Ritter Thu, 01/22/2004 - 08:49
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You are right on the 7200s. Without knowing the network it is hard to make any kind of recommendation but another solution could be to take router E and D out of the forwarding path.

Harold Ritter Thu, 01/22/2004 - 08:57
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One more thought. If this is a SP type of environment it would be worth considering running MPLS where D and E could be P routers and wouldn't need to maintain full routing table but just the IGP routes.

My two cents.


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