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Tuning BGP advertized routes

Unanswered Question
Jul 21st, 2013
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hey all.

I have two routers connecting to two upstream ISP's...(Im an ISP myself) one primary one secondary. The two routers have the exact BGP configuration. i recently configured local preference on the routers with one set to 150 and the other at default 100. now the problem is that the router with pref of 150 (R1) has started advertizing routes which are learned by (R2) how can i stop this? i do not want to advertize these routes. i only want to advertize my routes. my R1 is even getting to 100% cpu due to the same! now when i do a "show ip bgp nei xx.xx.xx.xx advertised-routes" I am advertising so many routes i cant even find the ones I want to advertize only! how can i stop these updates?

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Harold Ritter Sun, 07/21/2013 - 19:50
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Hi Amos,

It is recommended to filter the routes you advertise to your upstream SPs. You could tag the routes you received from your customers and your internal routes with a special community and only allow routes tagged with this community to be advertized to the upstreams SPs.


sharad1988 Sun, 07/21/2013 - 22:42
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Can you please put the BGP config section here of both router ?

ashirkar Mon, 07/22/2013 - 00:27
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Hello Amos,

In BGP you can filter routes on Neighbor based so you can control route received from one neighbor to another. Use route-map for it. Configure prefix list or access-list with permit for route you want to advertise to your neighbor and configure only single route map with permit statement and attached your ACL/prefix-list to it.

Example: If you want to advertise only prefix to your neighbor

step 1:Configure prefix list with permit statement

ip prefix-list Filter permit

step 2:Configure route-map

route-map permit Route_Filter_To_R2

match ip prefix-list filter

Step3: Attached route-map to BGP

conf t

router bgp x

nei x.x.x.x route-map Route_filter_To_R2 out ......as you want to filter route to R2 , use outbound filter.

Please also go through document

BGP Prefix-Based Outbound Route Filtering

There are many ways you can do filtering in BGP like using community-list, As-path list etc.

Hope this help you



Community Manager-NI


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