Subject: Re: l2vpn-vpls-multi-homing-01.txt

From: Bhupesh Kothari <[email-masked]>
Date: Sat, 02 Aug 2008 17:24:20 -0700

Hi Jim,

UTTARO, JAMES, ATTLABS <[email-masked]> wrote:

> The current approach being used to accomplish leaking "multi-homed"
> information between AS's is too tag the primary with some CV and
> then take action as it crosses the AS boundary ( set LP )..

This won't work for VPLS inter-AS option (b) if different providers
set different LP for an advertisement that belongs to a particular
site.  This would only work if each ASBR, in all ASes, is configured
to set the LP of each VPLS advertisement to what is configured on the
originating PE.  This would require a lot of provisioning.

Note that setting of LP on an ASBR is not a local AS matter as each
VPLS PE in all ASes need to see the exact same LP for an
advertisement.  This is a bit different than what you would usually do
with LP on an ASBR, say for internet routes.

> The draft seems to take the approach of mapping the LP to a VE
> preference for a route ( VE-ID) in the local AS ( AS1). The
> advantage of the first solution is that a route may be leaked across
> multiple AS boundaries. As the CV remains ( transitive ) I can
> always key off of it to set LP in the current AS domain. How can I
> accomplish this with VE pref? Is it kept as the route transits
> through multiple AS? I assume that VE pref is transitive and not
> reset upon re-advertisement from remote AS2 to remote AS3.

Yes, VE preference is transitive as it is carried in L2 Info community
and VE preference is not changed when advertisements are carried
across AS boundaries.  There is no reason to change the VE preference
on an ASBR and path selection can break if different PEs see different
VE pref. values.

The ASBR is translating LP to VE and VE to LP for backwards
compatibility only.

>
> It states for "BGP Path Selection" that ADs with the same exact <RD,
> VE-ID, VBO> are candidates for BGP Path Selection.. In our network
> we use unique RDs to reduce convergence time for primary/backup. Our
> approach is to ensure that both the primary and backup sites are
> opaque to the BGP topology. In this way both routes are sent to all
> PEs participating in the L2VPN. So how would a receiving PE treat to
> routes that have the same VE-ID but different RDs? The draft states
> the they would not be candidates for "BGP Path Selection". This
> implies I cannot manipulate LP to select a primary when I use unique
> RDs. This definitely is in conflict with our fast convergence goals
> for our L2VPN customers.

The draft states that such routes are not candidates for path
selection only for BGP path selection, and not for VPLS path
selection.  Such routes will be seen as different routes by RR as RD
is different, and that is what you want.  However, on the VPLS PE the
routes are candidates for VPLS path selection as VE ID is same, and
thus, VE/LP will be used to select the best route.

> Jim Uttaro

Bhupesh