Route election

Unanswered Question
Apr 7th, 2009
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3 sites, A,B,C connected with MPLS WAN


site A has network N1, site A CE use "network N1" under BGP to advertise it


Site B has static route, ip route N1 next hop xxxxx, SiteB CE BGP redistribute the static route


my question is


how Site C learn the the network N1, two routes?

how Site A reject/accept the route from siteB


Thanks


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Laurent Aubert Tue, 04/07/2009 - 17:32
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  • Cisco Employee,

Hi,


I assume you are in MPLS-VPN environment.

In this case:


1- For site C to learn N1, PE connected to site A and B needs to export N1 using route-target BGP extended community. PE connected to site C needs to import those routes. To be sure PE-C receives both routes, PE-A and PE-B should have a unique RD configured (in case there is a RR) and should both prefers their eBGP route.


2- It depends if you have the same BGP AS on site A and B and if the PEs are configured with the as-override feature. So we have the following scenarios:

- Different BGP AS: prefix accepted

- Same BGP AS with as-override: prefix accepted

- Same BGP AS without as-override: Prefix rejected.


HTH


Laurent.

study_voip Tue, 04/07/2009 - 17:37
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Thanks for your reply


it is MPLS-VPN, same BGP AS for all sites.


so siteC CE will learn two routes, one is learned from site A via "network N1", one is learned from site B via "redistribute", which one will take preference?


for site A, since it already have route to Network N1, so if site A learned one another route via site B(redistribute), how site A to make routing decision?


thanks again


Laurent Aubert Wed, 04/08/2009 - 05:39
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  • Cisco Employee,

Hi,


Site C CE will not learn two routes but only the best one selected by Site C PE which will have the two routes.


Site C PE will apply the BGP route selection algorithm to make its choice:


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml


If PEs are configured to do as-override, Site A CE will accept the update originated from Site B but will prefer by default its local network (see link below)


HTH


Laurent.

Laurent Aubert Wed, 04/08/2009 - 05:40
User Badges:
  • Cisco Employee,

Hi,


Site C CE will not learn two routes but only the best one selected by Site C PE which will have the two routes.


Site C PE will apply the BGP route selection algorithm to make its choice:


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml


If PEs are configured to do as-override, Site A CE will accept the update originated from Site B but will prefer by default its local network (see link below)


HTH


Laurent.

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