Re: [Idr] draft on virtual aggregation

Daniel Ginsburg <> Tue, 08 July 2008 21:21 UTC

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Date: Wed, 09 Jul 2008 01:21:36 +0400
From: Daniel Ginsburg <>
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Subject: Re: [Idr] draft on virtual aggregation
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Paul Francis wrote:
> Its posted by IETF now.  At


I think that there're few problems with the draft.

One is with PHP. If a router is a border VA router, it must perform 
*ultimate*-hop-popping, not penultimate as section 3.2.3 of the draft 
states, since the router is egress for an LSP (as per RFC5036 
terminology). I.e. it means that a border VA router must not advertise 
implicit (or explicit) null label for FECs corresponding to external 
next-hops. Also section 3.2.3 implies that external next-hops must be 
carried inside AS' IGP, and IMHO the draft should explicitly say so.

Another problem is mixing legacy hop-by-hop forwarding and VA. Consider 
the following topology.

|        |
|100     | 10
|        |
    10       10

A, B, D are VA routers. A is APR. B and E are legacy routers capable of 
only hop-by-hop forwarding, i.e. not doing MPLS. Link metrics are shown.

Packets ingress the topology at A. They get forwarded to B as plain IP 
since B is not an LSR. B forwards plain IP packets to D. D is a VA 
router and has suppressed the specific routes, thus it sends the packets 
back to A.

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