prepend aggregate ?

Unanswered Question
Mar 30th, 2009

Can anyone tell me if you can set up and aggregate route under BGP-let's say aggregate 192.168.65.0 255.255.255.0 summary-only and than do a route-map on BGP neighbour with as prefix-list matching 192.168.65.0 and setting a prepand for that ? Please let me know. so I guess my question is if you can prepend an aggregate

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Matt qomat Tue, 03/31/2009 - 06:22

but as-set will only show AS numbers that route came from. I need to prepend that route: example having two sites sending same aggregated router and you want prepend one of that aggregate. Both sites are in the same AS# and advertising aggregate to the cloud. So aggregate is showing same AS# from both. Adding as-set will not add additional AS. I want to add same AS three times so spoke sites will see one preferred shorter path.Please help

Laurent Aubert Mon, 03/30/2009 - 20:20

Hi,


I made a quick test and it's working.


setting the AS prepend via the attribute-map is not supported.


HTH


Laurent.

Matt qomat Wed, 04/22/2009 - 06:11

Did you configure something similar ?


router bgp 65555

no synchronization

bgp log-neighbor-changes

aggregate-address 192.168.0.0 255.255.0.0 summary-only

aggregate-address 192.168.10.0 255.255.255.0

aggregate-address 192.168.20.0 255.255.255.0

timers bgp 10 30

redistribute static

neighbor 10.0.0.2 remote-as 64552

neighbor 10.0.0.2 route-map prepend out

neighbor 10.1.0.2 remote-as 64552

neighbor 10.1.0.2 route-map prepend out

no auto-summary

!

ip http server

ip forward-protocol nd

ip route 192.168.10.0 255.255.255.0 Null0

ip route 192.168.20.0 255.255.255.0 Null0

!

!

!

!

ip prefix-list agregat seq 5 permit 192.168.0.0/16

ip prefix-list agregat seq 10 permit 192.168.10.0/24


route-map prepend permit 10

match ip address prefix-list agreg

set as-path prepend 65555 65555



It looks like router is taking only aggregate 192.168.0.0/16 and appends a prepend, which is what I wanted. Can you confirm ?

Matt qomat Wed, 04/22/2009 - 13:31

other question.

So site sending aggregate of two routes will keep advertising the aggregate even when it looses one of those routes?

like 192.168.0.0/16 when 192.168.10.0/24 is lost it will keep sending an aggregate until second routes is gone, right ?Will it send aggreagte after both are gone though?

Laurent Aubert Wed, 04/22/2009 - 16:46

Actually you don't need the following commands:


aggregate-address 192.168.10.0 255.255.255.0

aggregate-address 192.168.20.0 255.255.255.0


Because the routes you are redistributing are already /24 subnets (same as your aggregated addresses)


You need at least one more specific route in your BGP table for an aggregate to be valid and propagated. So if you remove one static route, your /16 will still be announced. If you removed both static routes, your aggregate will be withdrawn.


HTH


Laurent.

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