BGP neighbor question

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Oct 14th, 2007
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Hi.As I am a complete beginner at configuring BGP and have the following question.

Can you reduce the number of routes you receive on a router from another router that is in the same AS but connected to another ISP?

All the examples I have seen deal with reducing the size of routing table you receive from the ISP connected to the router.

If it's possible can you direct me to an example?

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gaurav_thapar79 Mon, 10/15/2007 - 01:15
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While filling BGP request form of your ISP, please mention your requirement of FULL routing/Partial Routing or default-route.

It actually depends on router Model and memory. Full routing option would give in excees of 2 lakh routes and would require in excess of 512 MB of RAM.

Recommended 1GB.

Kindly check with ur ISP n rate the post.

royalblues Mon, 10/15/2007 - 02:30
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You need to tell the ISP to filter the routes that are sent to you as per your requirements

Additonal you should configure an inbound policy at your routers that filters only required prefixes.

eg. to get only default routes

router bgp xxxx

neighbor a.b.c.d remote-as yyyyy

neighbor a.b.c.d route-map acceptroutes in

neighbor e.f.g.h remote-as ZZZZZ

neighbor e.f.g.h route-map acceptroutes in

access-list 3 permit

route-map acceptroutes permit 10

match ip address 3



ariesc_33 Mon, 10/15/2007 - 04:04
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when you say to reduce, you mean to say summarize? or filter some route? in case 1, sou should ask your ISP to aggregate some network.

gaurav_thapar79 Mon, 10/15/2007 - 05:34
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It does not happen like this. Every ISP runs their own iBGP and have their own Route-reflectors which actually control the length of routing table.Such routing tables actually maintains the best route available on internet to reach a specific destination.

You need to specify whether you want partial routing/Full routing or just single default-route on eBGP neighbour.

Now, you can apply your policy based routing to control your upstream/downstream traffic based on your decision policy.

If you are receiving defualt-route from your ISP , then putting permit any would not suffic any requirement since you have permitted and you are receiving the default-route itself with AD=20.

Please respond incase of further query.

Haris P Mon, 10/15/2007 - 11:42
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Use a prefix-list to filter according to the prefix-length ..

For Example the following prefix will allow only prefixes less than /29

ip prefix-list test seq 100 permit le 29

router bgp 1234

neighbor x.x.x.x prefix-list test in

i hope this is what u mean

rajatsetia Tue, 10/16/2007 - 05:04
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I think you original question is ,how to reduce the number routes from one router to another with in same AS running iBGP. and then you add the line but connected to another ISP, lead to some confusion..

are you trying to say something like this :-

one Router A is conected to ISP and receiving all the routes (eBGP) from ISP and in other direction router A is connected to router B in the same AS (iBGP) and you are asking if number of routes can be decreased from router A to router B ?

** you have already said that you have seen many examples reducing the number of routes from ISP.

i hope if you can explain your query better then you will get the exact response and if I have explained your question correctly then you can use many of techniques already explained by many pros even in case of iBGP e.g. using prefix list filtering..



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