draft-ietf-idr-as0-00 (bgp update destroying transit on redback routers ?)
dbg at net-geek.org
Fri Dec 2 07:12:48 CST 2011
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 protocols.
> 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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