MPLS BGP PE-CE multihomed: different RDs aren't enough ?

[apasquino]

Hello Community,

I know this may appear a tedious subject, but I can't find a clear clue, so I'm looking for your advice.

I have three PEs: a dual homed CE ia attached via PE1 and PE2 to a L3VPN (any-to-any, single RT) that run also on a "remote" PE3

RDs on PE1,2 are different, routes are injected via a route-map that manipulates local preference  and I have two RRs in the core. (LP-PE1 > LP-PE2)

Scenario 1: PE-CE static routing. From "remote" PE3 I receives the updates from PE1,2 for each static route with correct RDs and LPs 

Scenario 2: PE-CE eBGP routing. From "remote" PE3 I receive the updates only from a single local PE for each route that comes from the CE via eBGP. RDs and LP are correct, but I see only the update with the higher LP (from PE1). I see the same on the RRs, the prefix with the lower LP is withdrawn by PE2, just like it was overwritten by PE1. If eBGP PE1-CE dies, on PE3 i see again the correct update from PE2. If I use different RT between PE1 and PE2 evething is fine. What's the trick ?

Best Regards

Andrea

[sumit19851]

Hi Andrea,

when you are using single RT and eBGP with CE, PE2 also receives route from PE1 with higher LP and prefers higher LP route over eBGP router and withdraws route towards RR and PE3 receives only one route.

when you use static route, though PE2 receives route but it prefers static and advertises BGP route to RR.

If you use different RT, PE2 does not receive PE1's route and keeps advertising its route to RR. 

[apasquino]

Sumit,

I see your point. The question is: in a normal any-to-any L3VPN with a single RT across the PEs how can I achieve the goal of having a remote PE with 2 BGP "routes" ready for use ?

Andrea

[sumit19851]

Andrea,

In this case also backup path will work when active fails but if you want to see both routes on RR and remote PE then you need to prefer eBGP route over iBGP on PE2. which can be achieved by setting weight for ebgp neighbor which will be locally significant.

[apasquino]

Sumit,

You are right, setting weight on PE2 does the job. Unfortunately we can't use weight for policy reasons: we run a multivendor MPLS network, so I have to implement a solution which is good for any PE combination. If I do not "drug" LP the PEs and rely on AS_PATH, and again PE1 wins the update race over PE2 (updates from CEs come with a longer AS_PATH)

Andrea

[sumit19851 · Accepted solution]

Andrea,

Using different RT would be standard solution. Here problem is that on PE2 we want PE2 route to win and on PE3 we want PE1's route to win. Any attribute which is local to the router is weight. Other than this IGP metric to next-hop is one option but you need to increase metric of loopback of PE2 in such a way that PE3 will always prefer PE1. Then without doing any LP change PE3 can prefer PE1's route.

Sumit