Subject: Aggregation and MULTI_EXIT_DISC, NEXT_HOP

From: "Fryczke, Pawel" <[email-masked]>
Date: Thu, 4 Jul 2002 14:08:02 +0200

The latest draft, draft-ietf-idr-bgp4-17.txt states:
Routes that have the following attributes shall not be aggregated
unless the corresponding attributes of each route are identical:
MULTI_EXIT_DISC, NEXT_HOP.

Many vendors (e.g. Cisco/Juniper) does not follow this requirement. Why?
What is the purpose of this requirement?
Pawel

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From: Kunihiro Ishiguro <[email-masked]>
Date: Fri, 05 Jul 2002 20:33:18 -0700

>MULTI_EXIT_DISC, NEXT_HOP.

Well, this comes from RFC1771 and we didn't change it.

9.2.4.2 Aggregating Routing Information

   Aggregation is the process of combining the characteristics of
   several different routes in such a way that a single route can be
   advertised.  Aggregation can occur as part of the decision  process
   to reduce the amount of routing information that will be placed in
   the Adj-RIBs-Out.

   Aggregation reduces the amount of information that a BGP speaker must
   store and exchange with other BGP speakers. Routes can be aggregated
   by applying the following procedure separately to path attributes of
   like type and to the Network Layer Reachability Information.

IHMO, the intention of this part is "BGP should aggregate the routes
which have same network reachability".  Diffrent MULTI_EXIT_DISC or
NEXT_HOP means different network reachability.  So the specification
said "shall not be aggregate".

>What is the purpose of this requirement?

Today's network operator take control the use of aggregation.  So even
if we aggregate diffrent network rechability routes, that should be
ok.
-- 
Kunihiro Ishiguro

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From: Mathew Richardson <[email-masked]>
Date: Sat, 6 Jul 2002 02:05:54 -0400

> Kunihiro Ishiguro <[email-masked]> [Fri, Jul 05, 2002 at 08:33:18PM -0700]:

<snip>

The current mandate for the BGP spec. is to document current
implementations.  If vendors are currently aggregating routes
with different MEDs, or NEXT_HOPs, then this restriction should
be made a suggestion ("It is recommended that routes that have
differing MULTI_EXIT_DISC, or NEXT_HOP, attributes not be
aggregated"), or removed entirely.

mrr

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From: Kunihiro Ishiguro <[email-masked]>
Date: Sat, 06 Jul 2002 04:31:22 -0700

>aggregated"), or removed entirely.

I think so too.
-- 
Kunihiro Ishiguro

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From: Jeffrey Haas <[email-masked]>
Date: Mon, 8 Jul 2002 13:58:38 -0400

> aggregated"), or removed entirely.

It would be nice to know why the restriction was placed in the
specification in the first place before we suggest it be removed.

-- 
Jeff Haas 
NextHop Technologies

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From: Alex Zinin <[email-masked]>
Date: Tue, 9 Jul 2002 00:30:28 -0700

> specification in the first place before we suggest it be removed.

If we perform aggregation as part of the Decision process (as the
document says), it would mean that we're installing aggregated route
and not installing any specifics in our local RIB. The restriction
in question is to make sure it is safe to do so.

The question is whether anyone actually does this instead of
aggregating during the Update process...

Alex