[j-nsp] Usage of Route distinguisher in VPLS

[Kaliraj]

Suppose multiple sites end up using the same Site-id (VE-id) but
participate in different VPLSs(differnent RTs) then RD will help
distinguishing the layer-2 bgp-nlri for the layer2-routes for these
sites. Isn't it?

Else (if RD is same or non-existent), advertisement for one site will
mask the other's - assuming site-id, block-offset etc are same.

Kaliraj
On 6/27/07, Monika M <monika.vpls at gmail.com> wrote:
> Thanks for the reply.
> As per section 3.1.2, Route target only identifies the VPLS.
> Even in Juniper, if I have a configuration like this,
>
> root# show routing-instances
> vplsinst2 {
>    instance-type vpls;
>    route-distinguisher 100:4;
>    vrf-target {
>        import target:100:3;
>        export target:100:3;
>    }
>    protocols {
>        vpls {
>            site-range 15;
>            site india {
>                site-identifier 7;
>            }
>        }
>    }
> }
> vplsinstance {
>    instance-type vpls;
>    interface fe-0/3/1.0;
>    route-distinguisher 100:3;
>    vrf-target {
>        import target:100:3;
>        export target:100:3;
>    protocols {
>        vpls {
>            site-range 10;
>            site vplsSiteA {
>                site-identifier 6;
>                interface fe-0/3/1.0;
>            }
>        }
>    }
> }
>
> If a NLRI is received with RD as 100:3 and RT as 100:3, it is added to both
> the VPLS instances.
> Route tables vplsinst2.l2vpn.0 and vplsinstance.l2vpn.0 contain an entry for
> the single NLRI.
>
> If the RT configuration is different and RD is same, then the NLRI is
> associated only with the VPLS instance which is having the received RT.
> Hence VPLS association is basically done with the RT and not with RD. That
> is why it looks to me that RD is a redundant field.
>
>
> Another feedback for the multi-homing input.
>  Having different RDs will lead to duplicate packets and loops (if you don't
> run STP) as you treat them as two different prefixes and PWs are established
> to both the peers leading to problem. I believe it is similar to having
> different VE ID configuration stated in the RFC.
>
>
> -Monika
>
> On 6/27/07, Guy Davies <aguydavies at gmail.com> wrote:
> >
> > Hi Monika
> >
> > On 27/06/07, Guy Davies <aguydavies at gmail.com> wrote:
> > > Hi Monika,
> > >
> > > I haven't played around much with VPLS but a quick read of RFC 4761
> > > reveals the following...
> > >
> > > Section 3.5 of RFC 4761 appears to be the key.  It refers to
> > > multihoming mechanisms for VPLS and states the following...
> > >
> > >    In the case where the PEs connected to the same site are assigned the
> > >    same VE ID, a loop-free topology is constructed by routing
> > >    mechanisms, in particular, by BGP path selection.  When a BGP speaker
> > >    receives two equivalent NLRIs (see below for the definition), it
> > >    applies standard path selection criteria such as Local Preference and
> > >    AS Path Length to determine which NLRI to choose; it MUST pick only
> > >    one.  If the chosen NLRI is subsequently withdrawn, the BGP speaker
> > >    applies path selection to the remaining equivalent VPLS NLRIs to pick
> > >    another; if none remain, the forwarding information associated with
> > >    that NLRI is removed.
> > >
> > >    Two VPLS NLRIs are considered equivalent from a path selection point
> > >    of view if the Route Distinguisher, the VE ID, and the VE Block
> > >    Offset are the same.  If two PEs are assigned the same VE ID in a
> > >    given VPLS, they MUST use the same Route Distinguisher, and they
> > >    SHOULD announce the same VE Block Size for a given VE Offset.
> > >
> > > In section 3.3, it states (paraphrasing) that you MAY configure an RD
> > > on a PE for each VPLS.  If you choose not to configure one manually
> > > then one will be automatically created for each VPLS.
> > >
> > > So, (taking 3.3 and 3.5 together) if you want to do multihoming, you
> > > MUST manually configure the same RD for the same VPLS on all the PE to
> > > which multihomed sites in that VPLS are connected (so it makes sense
> > > to configure the same RD on all PE for that VPLS).
> >
> > Sorry, I should say the following...
> >
> > So, (taking 3.3 and 3.5 together) if you want to do multihoming *and
> > not use STP*, you MUST manually configure the same RD for the same
> > VPLS on all the PE to which multihomed sites in that VPLS are
> > connected (so it makes sense to configure the same RE on all PE for
> > that VPLS).
> >
> > It has been pointed out to me that using the same RD will have an
> > impact on the speed of convergence.  Because the two prefixes are
> > viewed as identical, the receiving PE will discard/ignore the other
> > 'identical' advertisements.  It must see a withdrawal of the first
> > prefix and then an advertisement of the originally discarded/ignored
> > prefix before the new prefix will be used.  If the RDs (and hence the
> > prefixes) are different, then only the withdrawal of the preferred
> > prefix is required to trigger the use of the other prefix.
>
>
>
>
>
> Rgds,
> >
> > Guy
> >
> _______________________________________________
> juniper-nsp mailing list juniper-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/juniper-nsp
>