[c-nsp] BGP Signalled VPLS

Bruce Pinsky bep at whack.org
Wed Apr 24 12:31:47 EDT 2013

Hash: SHA1

Caillin Bathern wrote:
> Aaron,
> I think bep is referring to EVPN in his comment.  In BGP signalled + BGP
> AD VPLS, you don't use a specific split-horizon label.  Instead if you
> think of the most simple case where you have three PEs: PE-A, PE-B and
> PE-C.  PE-A (better local pref)and PE-B (worse local pref) are
> multi-homed to site CE-A and PE-C is single-homed to site CE-C.
> Now if you consider the BGP advertisements received at PE-C, it has two
> possible routes to site CE-A (distinguished by the identical RD, VE ID
> and VE block offset).  Hence PE-C selects one of these two sites as its
> route to CE-A via BGP route selection process, being via PE-A because of
> local-pref.  Now PE-C sets up a single BGP signalled pseudo-wire between
> PE-C to PE-A and begins forwarding traffic.  Any traffic received at
> PE-B is dropped because it has no pseudo-wires set up to any other PEs
> and there is no loop in the network.
> J* have a good article describing exactly that here
> http://www.juniper.net/techpubs/en_US/junos/information-products/topic-c
> ollections/nce/bgp-vpls-multihoming/validating-a-bgp-based-vpls-multihom
> ing-configuration.pdf

Yes, with the cavaet that you are using the same VE ID on the two PE's
connected to the same site.  If you don't, then you must provide some loop
prevention mechanism.  So it is still possible to shoot yourself in the foot.

> EVPN is a little different because in that above example, PE-C would
> have set up pseudo-wires to both PE-A and PE-B allowing active-active
> forwarding.  Any BUM traffic will still be forwarded using split-horizon
> though using the split-horizon label.  At least that is my
> understanding..

Exactly.  Loop prevention is inherent in the implementation due to the
active-active capability.

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