Manipulate Default route

Unanswered Question
Sep 24th, 2008

Setup: I have 2 PE routers. PE-a and PE-b both connected to CE's(setup with EIGRP)that are advertising default routes to PE-c and currently PE-c is preferring PE-b due to natural iBGP selection process. Currently PE-c is running EIGRP with the CE as well. Is there a way I can manipulate the incoming default route at the VRF address family level on PE-c so that it would prefer PE-a. Basically I'm trying to force PE-c to prefer PE-a even though it prefers PE-b. Any help would be appreciated. Thanks!

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Harold Ritter Wed, 09/24/2008 - 17:34

Henry,

When you run eigrp as a PE-CE protocol, PE-c should not make its BGP best path decision as usual but rather compares the BGP cost community before everything else. Changing the eigrp metric(s) between PE-b and its CE to make it the preferred metric should cause PE-c to select PE-b instead of PE-a.

Regards

hks10 Thu, 09/25/2008 - 08:15

Thanks for the info but I was wondering if you can assist with a different scenario. If I had another location for example on PE-d and I wanted that location to use PE-a as a default gateway and PE-c to use PE-b, with the metric being that PE-b is the preferred route, how would I go about manipulating the PE-d router to choose PE-a instead of PE-b. Sorry I'm jumping around a lot but I'm basically trying to figure out how I can config my PE to pick and choose which default route to take manually instead of dynamically. I still need the dynamic part there just incase there is a failure but the manual part is there to have options. Any help would be appreciated. Thanks!

Harold Ritter Fri, 09/26/2008 - 11:57

Henry,

You could allow the different PEs to select the best path to the default route based on the best IGP metric from one PE to another (normal BGP best path selection process) but for that you would need to tell BGP to ignore the Cost community using the following command:

router bgp xx

bgp bestpath cost-community ignore

Regards

hks10 Mon, 09/29/2008 - 08:32

Is there a way I can do this on a specific VRF only? I have many different VRF's running on this same PE and I don't want to affect their traffic.

Harold Ritter Mon, 09/29/2008 - 17:24

Henry,

Disabling the evaluation of the cost commuity is global and would apply to all the VRFs. Are all VRFs running eigrp as PE-CE routing protocol?

Regards

hks10 Mon, 09/29/2008 - 21:41

Actually no. I'm only trying to manipulate the route for only 1 specific VRF.

Harold Ritter Tue, 09/30/2008 - 04:51

Henry,

In this case, you could assign a different loopback interface address for that VRF on the egress PE (i.e. "bgp next-hop loopback xx" under "ip vrf xxx") and then use a TE tunnel on the ingress PE for that specific loopback address to prefer one egress over the other. This would make it such that no other VRF would be impacted by the change.

Regards

hks10 Tue, 09/30/2008 - 10:49

Thanks for the info. I'm new to Traffic Engineering, I get the egress PE part but I'm a little confused on the ingress. I know how to create a Tunnel but after I create the tunnel how do I config the tunnel to prefer one route over the other?

Giuseppe Larosa Fri, 10/03/2008 - 10:41

Hello Harold,

in this case a route-map in AF vpnv4 or an export-map could raise the local preference for the PE to be preferred ?

Is this a possible solution ?

Best Regards

Giuseppe

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