[c-nsp] BGP Signalled VPLS

Wed Apr 24 10:03:04 EDT 2013

>Alright so once the VE ID and offset is the same on two PEs, the remote
PEs will create PW only to one of them. 
>This is taken care of the standard VPLS split-horizon rule though.
Not so - if PE-A and PE-B are not using the same VE ID/offset then they
will create a PW between themselves.  This then creates a loop
CE-A---PE-A---PE-B---CE-A.  No split-horizon involved there.  Similarly
if PE-C has active PW to both PE-A and PE-B then when it sends a BUM
frame across the VPLS, the CE-A will receive the BUM frame twice.

>What about PW between PE-A and PE-B (is that PW not created if these
two have now the same VE ID and offset?
Correct
>So that it solves the looping of BUM traffic originated at CE-A sent to
PE-A sent to PE-B and sent to CE-A. 
>But it affects every other PE in the domain as well. 
>So I'd cripple the whole VPLS domain in order to avoid loop between two
PEs, I guess I don't like that.
How does it cripple the VPLS, what do you mean by that?
It provides you an active-backup link to the CE at that site with no
requirement for xSTP.

Caillin

-----Original Message-----
From: Adam Vitkovsky [mailto:adam.vitkovsky at swan.sk] 
Sent: Wednesday, 24 April 2013 11:50 PM
To: Caillin Bathern; cisco-nsp at puck.nether.net
Subject: RE: [c-nsp] BGP Signalled VPLS

Alright so once the VE ID and offset is the same on two PEs, the remote
PEs will create PW only to one of them. 
This is taken care of the standard VPLS split-horizon rule though. 

What about PW between PE-A and PE-B (is that PW not created if these two
have now the same VE ID and offset? 
So that it solves the looping of BUM traffic originated at CE-A sent to
PE-A sent to PE-B and sent to CE-A. 
But it affects every other PE in the domain as well. 
So I'd cripple the whole VPLS domain in order to avoid loop between two
PEs, I guess I don't like that. 

I'd rather relay on MC-LAG or REP or MST access gateway in order to
avoid loops. 

adam
-----Original Message-----
From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
Caillin Bathern
Sent: Wednesday, April 24, 2013 2:54 PM
To: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] BGP Signalled VPLS

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

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..

Caillin

-----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.

Aaron

-----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

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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
that?
> 
> 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.

- --
=========
bep

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