Subject: [Idr] BGP best-path selection in confederations (draft-ietf-idr-rfc3065bis-06.txt)

From: "Ilya Varlashkin" <[email-masked]>
Date: Tue, 12 Jun 2007 17:29:50 +0200

Hello all,

I realise that now it's too late for significant changes in
draft-ietf-idr-rfc3065bis-06.txt, so I send this as standalone message.

Currently major implementations disregard length of AS_CONFED_SEQUENCE,
and such behaviour documented in step 3 of section 5.3 in
draft-ietf-idr-rfc3065bis-06. While MED or community+localpref can be
used to alter route preferences where only in-confederation path
different, I believe it will be advantageous to consider length of
AS_CONFED_SEQUENCE too as this can provide more clear and simple way of
choosing optimum route if path length within confederation varies.
Possible order for using AS_CONFED_SEQUENCE as tie-breaker could be
after MED and before IGP cost, i.e. somewhere between 'c' and 'e' in
section 9.1.2.2 of RFC4271). To avoid inconsistency with existing
implementation such change in best-path selection could be done via a
configuration option with default behaviour being as described in
RFC4271 and draft-ietf-idr-rfc3065bis-06.

Besides implementation work, are there other reasons not to use
AS_CONFED_SEQUENCE length as tie-breaker?

Cheers,
iLya

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From: Jeffrey Haas <[email-masked]>
Date: Tue, 12 Jun 2007 16:30:00 -0400

Ilya,

> AS_CONFED_SEQUENCE length as tie-breaker?

As long as the tie-breaking is consistent within the AS, it'll work
fine.  Perhaps a more interesting alternative to AS_PATH length
including confederation segments is to use confederation segment length
only when non-confederation lengths are comparable.

-- Jeff

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From: Jeffrey Haas <[email-masked]>
Date: Tue, 12 Jun 2007 17:03:56 -0400

Responding to myself:

> only when non-confederation lengths are comparable.

Doing this is usually pretty comparable to your IGP metric and may be
useful only when you have disjoint IGP domains.

-- Jeff

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From: "Ilya Varlashkin" <[email-masked]>
Date: Wed, 13 Jun 2007 00:10:13 +0200

Jeff, 

> -----Original Message-----
> As long as the tie-breaking is consistent within the AS, 
> it'll work fine.  Perhaps a more interesting alternative to 

I've encountered practical multi-vendor network scenario where proposed
path-selection modification would be benifical. Posting to IETF instead
of directly to a single vendor so that feature is consistent between
different implementations. Hopefully there are more people on the list
who will find this proposal useful.

> AS_PATH length including confederation segments is to use 
> confederation segment length only when non-confederation 
> lengths are comparable.
> 

Indeed. And that's why I mentioned doing this comparisson after MED and
before IGP cost, i.e. at that point non-confed AS-PATH ensured to be
equal. Whether it's done before eBGP/iBGP tie-breaking doesn't seem to
be essential.

Cheers,
iLya

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From: "Ilya Varlashkin" <[email-masked]>
Date: Wed, 13 Jun 2007 00:13:35 +0200

> > including confederation segments is to use confederation segment 
> > length only when non-confederation lengths are comparable.
> 
> Doing this is usually pretty comparable to your IGP metric 
> and may be useful only when you have disjoint IGP domains.
> 

That's exactly the case - each member AS runs its own IGP and I'm not
aware of any implementation that currently can _add_ IGP cost to MED,
only overriding MED with IGP cost, which may create incomparable MED's.

/iLya