From: Peter van Oene (pvo@usermail.com)
Date: Tue Sep 10 2002 - 10:55:06 GMT-3
comment inline
At 09:51 AM 9/10/2002 +1000, Nick Shah wrote:
>This is the standard behaviour of aggregation. The aggregate route is
>advertised to come from the 'aggregator as' (the AS of the router, which
>performed aggregation) and the atomic aggregate is set which basically means
>that 'some info is lost'. However if you wish to preserve the 'number of
>AS's from which individual prefixes have been derived' use the as-set
>keyword. So in ur case AS 2 performed aggregation, and so the route is seen
>with as-path of 2.
Of note, the default behavior is not standard per the spec. You are
supposed to automatically inject an AS-SET inclusive of all AS's seen in
contributing routes which Cisco doesn't do by default. Cisco certainly
provides the ability to do so, but its default is different from the spec
as well as some other BGP implementations which has caught folks a few
times in my experience.
Pete
>With the aggregate command, there are other options as well that you might
>want to look at :
>
>summary-only - advertises summary and suppresses more specific routes
>suppress-map - you can choose which prefixes to suppress
>advertise-map - you can 'select' which prefixes to use, when creating the
>aggregate
>attribute-map - you can specify specific attributes of the aggregate route
>(like metric, origin, community etc.)
>
>rgds
>Nick
>
>
> > I have an aggregated route (172.16.0.0/22) advertised from routerB to
>routerA.
> > A brief config as follows.
> >
> > Question:
> > 1) In 'show ip bgp' on routerB and routerA, the aggregate route
>172.16.0.0/22
> > has no Metric value, why is that?
> > 2) The 'show ip bgp 172.16.0.0 255.255.252.0 ' on routerA shows
> > 'atomic-aggregate', yet, 'show ip bgp' on routerA still shows path '2 i'
>for
> > this route. I thought the 'atomic-aggregate' means AS information for the
> > aggregate has been lost?
> >
> > RouterB#
> > router bgp 2
> > network 172.16.0.0 mask 255.255.255.0
> > network 172.16.1.0 mask 255.255.255.0
> > network 172.16.2.0 mask 255.255.255.0
> > network 172.16.3.0 mask 255.255.255.0
> > aggregate-address 172.16.0.0 255.255.252.0
> > neighbor 10.1.1.1 remote-as 1
> >
> > RouterA#
> > router bgp 1
> > neighbor 10.1.1.2 remote-as 2
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