[c-nsp] BGP Signalled VPLS

Caillin Bathern caillinb at commtelns.com
Wed Apr 24 08:54:11 EDT 2013


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

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


-----Original Message-----
From: Aaron [mailto:aaron1 at gvtc.com] 
Sent: Wednesday, 24 April 2013 3:42 AM
To: bep at whack.org
Cc: Caillin Bathern; cisco-nsp at puck.nether.net; adam.vitkovsky at swan.sk;
'Saku Ytti'
Subject: RE: [c-nsp] BGP Signalled VPLS

Well why didn't you just say so!  Haha, Seriously, thanks a bunch bep.
I'll begin reading up on this.


-----Original Message-----
From: Bruce Pinsky [mailto:bep at whack.org]
Sent: Tuesday, April 23, 2013 12:19 PM
To: Aaron
Cc: 'Caillin Bathern'; cisco-nsp at puck.nether.net;
adam.vitkovsky at swan.sk; Saku Ytti
Subject: Re: [c-nsp] BGP Signalled VPLS

Hash: SHA1

Aaron wrote:
> Thanks Caillin/Saku/Adam, this differentiation of VPLS LDP Sig 
> compared to BGP Sig as it relates to loop prevention during redundant 
> pe/ce at edge is of interest to me...(I actually had a l2 forwarding 
> loop scare me to death and had to shut down backside c-to-c during 
> maintenance window a few months ago)....i walked away from that with a

> big question in my head as to how does customer spanning tree feed 
> into the loop prevention of split horizon groups within a vpls as how 
> pw forwarding treatment occurs...and I thought to myself , it probably

> doesn't... which has had me wondering about this for a few months....
> BUT, now y'all mention that bgp signaled vpls as it relates to 
> redundant pe/ce avoids this.... correct?  I have adam's link, thanks 
> adam, but does anyone have more links related to understanding all
> Adam's implementation link....
> http://www.cisco.com/en/US/docs/routers/asr9000/software/asr9k_r4.3/lx
> vpn/co
> nfiguration/guide/lesc43pbb.html#wp1183684

By the selection of a Designated Forwarder via the Ethernet AD route
advertisement and the use of a split-horizon label appended to
multi-destination packets.

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