The MPLS-OPS Archive

[Yakov Rekhter <yakov@juniper.net>]

Jim,

> One point to add below ..

likewise, I have one point to add as well :-)

[clipped...]

> > >>So, why BGP instead of LDP?  BGP does auto-discovery; LDP doesn't.
> > >>This makes a huge difference to the task of provisioning VPNs.
> > >>
> > >>BGP is also better suited to building largish VPNs -- with LDP, the
> > >>protocol state maintained in the network, the number of targeted LDP
> > >>sessions, etc. all get pretty large pretty quickly [O(N**2)].  With
> > >>BGP, the protocol state in the network is O(N); and one can use route
> > >>reflectors and/or confeds to keep the number of sessions at O(N).
> > >>Finally, if one is interested in building inter-AS/inter-provider
> > >>VPNs, BGP is a clear winner.
> 
> I do not think that this is comparing apples to apples - directed LDP has to
> distribute information to a particular endpoint for a given attachment VC
> whereas BGP wants, by default, to distribute the information to everyone. So
> although there will be a larger number of directed LDP sessions than BGP
> sessions (with RRs), the amount of information that must be carried by the
> directed LDP sessions is by no means the same as BGP sessions. This means
> that the scaling implications are not the same as with BGP. Jim

To avoid distributing the information to everyone one can use BGP
Outbound Route Filters (ORFs). And with ORFs the amount of information
that one needs to carry over BGP is comparable to the amount of
information one would need to carry over the directed LDP session.

Yakov.

P.S. For more details on ORF one may look at 
draft-ietf-idr-route-filter-06.txt. 

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