You would think metric/AS_PATH manipulation would help, but nope. Here's my understanding of what's going on.
PE1 -------- PE2 | | | | CE1 -------- CE2 Running MP-BGP between PE1 and PE2 for MPLS VPN services. Running BGP between CE1 and CE2 as the internal routing protocol. PE1 is eBGP peer with CE1. PE2 is eBGP peer with CE2. PE1 is iBGP peer with PE2. CE1 is iBGP peer with CE2. In this configuration, the path between CE1 and CE2 will always be preferred. You would THINK that the eBGP path into the MPLS cloud would be preferred (eBGP > iBGP), but it's not. CE1 injects a route into BGP. It is advertised to CE2 via iBGP, which installs it. CE1 and CE2 both advertise the route to their eBGP neighbors of PE1 and PE2. Both PE routers install the route and advertise it to one another. The gotcha is what happens now. The routes shared between the PEs is iBGP. Since eBGP > iBGP, both PE routers prefer the routes towards their respective CEs. The result is that the CE routers share the prefix between them, but neither receives it from their PE neighbor. The only path the CEs can take is via the path between them. No amount of metric manipulation can choose a route that the CEs are not receiving. Now, if the CE1 to CE2 link DOES go down, CE2 will end up learning the route via the MPLS cloud...once the route propagates. PE2 will have to recalculate BGP, decide to prefer the (now only) path towards PE1 to get to the prefix, then pass the route on to CE2. Regardless, I don't know of any way (besides doing a lot of manual route filtering based on which routes are in the routing table) of making this setup prefer the MPLS path and use the CE1 to CE2 link only as backup. This is my understanding, feel free to tell me how wrong I am. =) Keller Giacomarro [email protected] On Mon, Oct 8, 2012 at 1:09 PM, Antoine Monnier <[email protected]>wrote: > I would have thought that you should be able to influence the PEs' > decision using the usual tool such as AS-path prepending, MED, etc > > I haven't labed it but the scenario you describe does not seem > different than the case where you have dual CEs at the same site but > you want all traffic to enter the site from one specific CE, and we > use as-path prepending on the backup CE to handle that. > In your case, each CE should prepend the routes from the other CE > towards its own PE. > > Then configure your CEs to prefer the routes through the MPLS backbone > by setting local-preference inbound when routes are received from the > PEs. At this stage, each CE will stop advertising the other CE's > prefix towards the PE since it itself choose the path towards the PE > as the Best Path. > > > > > On Mon, Oct 8, 2012 at 10:17 AM, Keller Giacomarro <[email protected]> > wrote: > > Right -- no pure BGP solution? > > > > Keller Giacomarro > > [email protected] > > > > > > On Mon, Oct 8, 2012 at 3:16 AM, WFT <[email protected]> wrote: > > > >> > >> Two sites, same ASN, with backdoor (backup-only) link > >> - ?? > >> > >> > use sham-links with ospf > >> > use bgp extended cost community with eigrp > >> > >> Sent from my iPhone > >> > >> On Oct 8, 2012, at 5:18 AM, Keller Giacomarro <[email protected]> > wrote: > >> > >> > Two sites, same ASN, with backdoor (backup-only) link > >> > - ??? > >> > > _______________________________________________ > > For more information regarding industry leading CCIE Lab training, > please visit www.ipexpert.com > > > > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com > > > > http://onlinestudylist.com/mailman/listinfo/ccie_rs > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
