From: Nick Shah (nshah@connect.com.au)
Date: Mon Sep 09 2002 - 20:51:40 GMT-3
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.
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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