Subject: [Idr] Atomic Aggregate

From: Tulip Rasputin <[email-masked]>
Date: Tue, 8 Jun 2004 06:56:57 -0400 (EDT)

Hi,

Section 5.1.6 (ATOMIC_AGGREGATE) of the draft-ietf-idr-bgp4-23.txt states that "If an aggregate
excludes at least some of the AS numbers present in the AS_PATH of the routes that are aggregated
as a result of dropping the AS_SET, the aggregated route, when advertised to the peer, SHOULD  
include the ATOMIC_AGGREGATE attribute."

Now suppose a BGP speaker (in AS say XXX) has the following routes:

[1] 128.1/16, AS Path Sequence 6000 6120
[2] 128.2/16, AS Path Seq 6000 

This BGP speaker is configured to advertise an aggregate route of 128/8 to one of its EBGP Peer.

If this BGP speaker advertises 128/8 with AS Path "XXX 6000", then IMO it should attach the
ATOMIC_AGGREGATE path attribute, warning its peer that some of the AS numbers are not present in
this AS Path.

What would happen if it advertises 128/8 with AS Path Seq "XXX 6000"  and AS Set "[6120]"? In this
case its advertising all the AS Path information. Is it required to attach the ATOMIC_AGGREGATE
path attribute in this case?

I understand that it will attach the AGREGATOR path attribute in both the above cases to let
others know who and where did the aggregation took place?

Regards,
Rasputin

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From: Jeffrey Haas <[email-masked]>
Date: Tue, 8 Jun 2004 08:25:28 -0400

> path attribute in this case?

All of the AS's are present, even if not attached to their original
sequences.  Since BGP uses the presence of the AS, regardless of its
type (sequence, set, etc) for loop detection, this is enough. 

ATOMIC_AGGREGATE doesn't need to be attached.

-- 
Jeff Haas 
NextHop Technologies