[j-nsp] Usage of Route distinguisher in VPLS

[Guy Davies]

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  If the chosen NLRI is subsequently withdrawn, the BGP speaker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