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Simple Question for the BGP Experts

wpalumbo06
Level 1
Level 1

So, I have two routers at the edge of a Data Center, both with eBGP peerings to the same MPLS service provider.  Both routers are configured to advertise the networks contained in the Data Center.  Due to the configurations on the routers and the fact that the MPLS provider has as-override enabled, R1 is advertising the prefixes correctly but R2 is not.  R2 is learning the Data Center prefixes via external EIGRP and via eBGP from R1 (via the service provider).  I know why this happening and how to fix but I am curious as to why R2 is not advertising any of the Data Center prefixes?  It seems logical that it would not since it's learning these prefixes from the provider but this would seem to display the behavior of split horizon and my understanding is that split horizon is only used for iBGP...am I missing something obvious here?  Both Data Center routers are using the same ASN but are NOT peered with each other - that's actually the fix for this issue but has not yet been implemented.

2 Replies 2

chrihussey
VIP Alumni
VIP Alumni

Seems the easiest thing (and a common practice) would be to be to put a filter in place denying advertisement of the data center networks to R2 from the provider. It should also be applied to the R1 router.

IBGP should be make things more manageable but don't think it solves this issue.

 Also, not sure of your setup, but once both routers are advertising to the MPLS provider, be aware of the possibility of asymmetric routing. Out R1, coming back via R2 or vice versa. If there are multiple firewalls, etc, this could be a problem.

Hope this helps.

IBMeR
Level 1
Level 1

I don't know exactly what happened - there is not enough information.

But I may guess:

1) eBGP has more preffered administrative distance then EIGRP.

2) In RIB were installed eBGP routes.

3) EIGRP routes were received, but not redistributed in BGP, because they didn't installed in RIB.

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