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Re: [Idr] draft-ietf-idr-as0-00 (bgp update destroying transit on redback routers ?)
From: Brian Dickson <brian.peter.dickson () gmail com>
Date: Sat, 3 Dec 2011 12:22:56 -0500

Can anyone familiar with this knob and its usage, answer a question:

Would anything break, in terms of use of that knob, if instead of
"zeroing" the AGGREGATOR, the local AS (as seen from the outside
world, in the case of confederations) were used?

Would the functionality of the knob, in reducing updates, be preserved?

Would routes be considered malformed or would it trigger any other bad behavior?

Perhaps this is a way of resolving the conflict between this knob and
the AS0 draft?


On Fri, Dec 2, 2011 at 8:12 AM, Daniel Ginsburg <dbg () net-geek org> wrote:

This is true that "no-aggregator-id" knob zeroes out the AGGREGATOR attribute.

The knob, as far as I was able to find out, dates back to gated and there's a reason why it was introduced - it helps 
to avoid unnecessary updates. Assume that an aggregate route is generated by two (or more) speakers in the network. 
These two aggregates differ only in AGGREGATOR attribute. One of the aggregates is preferred within the network (due 
to IGP metric, for instance, or any other reasons) and is announced out. Now if something changes within the network 
and the other instance of the aggregate becomes preferred, the network has to issue an outward update different from 
the previous only in AGGREGATOR attribute, which is completely superfluous.

If the network employs the "no-aggregator-id" knob to zero out the AGGREGATOR attribute, both instances of the 
aggregate route are completely equivalent, and no redundant outward updates have to be send if one instance becomes 
better than another due to some internal event, which nobody in the Internet cares about.

In other words, the "no-aggregator-id" knob has valid operational reasons to be used. And, IMHO, the 
draft-ietf-idr-as0-00 should not prohibit AS0 in AGGREGATOR attribute.

On 02.12.2011, at 1:56, Jeff Tantsura wrote:


Let me take it over from now on, I'm the IP Routing/MPLS Product Manager at Ericsson responsible for all routing 
There's nothing wrong in checking ASN in AGGREGATOR, we don't really want see ASN 0 anywhere, that's how 
draft-wkumari-idr-as0 (draft-ietf-idr-as0-00) came into the worlds.

To my knowledge - the only vendor which allows changing ASN in AGGREGATOR is Juniper, see "no-aggregator-id", in the 
past I've tried to talk to Yakov about it, without any results though.
So for those who have it configured - please rethink whether you really need it.

As for SEOS - understanding that this badly affects our customers and not having draft-ietf-idr-error-handling fully 
implemented yet, we will temporarily disable this check in our code.
Patch will be made available.

Please contact me for any further clarifications.


P.S. Warren has recently  included AGGREGATOR in the draft, please see

2. Behavior
  This document specifies that a BGP speaker MUST NOT originate or
  propagate a route with an AS number of zero.  If a BGP speaker
  receives a route which has an AS number of zero in the AS_PATH (or
  AS4_PATH) attribute, it SHOULD be logged and treated as a WITHDRAW.
  This same behavior applies to routes containing zero as the
  Aggregator or AS4 Aggregator.

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