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BGP with two ISP


I have CCNP but unfortunately dont have real experiance with BGP. We have some issue in my company.  We have 2 BGP peer with 2 ISP, they advertise to us full BGP table, but because our core router is very old, we dont have enough memory to have 2 full bgp table. (We'll replace him soon, but what is soon, maybe 2-3 month, or one year :) , so we need some solution now.

First, i think that with one ISP we have full BGP table (primary), and with other only def. route (secondary). Is this feasible in real world ?

Can i do something wit BGP atribibutes like AS PATH, and say that i only want inbound route from first ISP ?

Please, if you have any suggestion what to do, what is best practise, i would like to hear that.

Thanks and i wish all the best :)

Srdjan Raicevic

4 Replies 4


there are many ways to achieve your objective. The easiest would probably be to use a prefix list filter on your secondary ISP that allows only the default route:

ip prefix-list DEFAULT_ONLY seq 5 permit

neighbor xx.xx.xx.xx prefix-list DEFAULT_ONLY in


George has provided a good solution, you can use route-map to define which networks you want to receive or just a default network provided by the ISP, it will help you to decrease the RIB and it will use less CPU utilization. 



ip prefix-list PREFIXES seq 5 permit

ip prefix-list PREFIXES seq 5 permit

ip prefix-list PREFIXES seq 5 permit


route-map ISP1 permit 5
match ip address prefix PREFIXES

set local-preference 1000


route-map ISP1 deny 100


(ISP2 for redundancy)


route-map ISP2 permit 5
match ip address prefix PREFIXES

set local-preference 500


route-map ISP2 deny 100


router bgp X

neighbor remote Y

neighbor route-map ISP1 in

neighbor remote Z

neighbor route-map ISP2 in


clear ip bgp * soft 


* Also you could include AS-PATH attribute to have symmetric traffic.


Hope it is useful


>> Marcar como útil o contestado, si la respuesta resolvió la duda, esto ayuda a futuras consultas de otros miembros de la comunidad. <<


What you dont mention is this just one rtr with two ISP peerings or two rtrs connected together via an IGP or IBGP with an ISP peering each?


BGP PA attribute manipulation is the way to go but it would all depend what topology you have to what is best for you implement , but one thing is certain, you need to make sure you DO NOT become a transit path for either ISP, that would be very detrimental to your site.


Please rate and mark as an accepted solution if you have found any of the information provided useful.
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Kind Regards




Just to add to this, If you are planning on receiving full routes via ISPA and then a default route via ISPB then ISPB will never be used for egress traffic as the full routes via ISPA will be more specific. If you are happy with this, and want to use ISPA as primary and ISPB as backup for egress traffic, then its probably not worth receiving full routes from either ISP and request that they both advertise you with a default route. You can then use BGP local-preference to prefer one default over the other.


Hope this helps

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