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Problem with BGP

Plz help on BGP, I am working for the ISP. I have Many Backbones, and few Block of Class C ( 195.X.X.X )IPs. Here i am advetising the each block to separate backbones throu rote-maps In BGP How can we split the one Block. For Ex: 195.X.X.0 has to come throu one Backbone and Remaining has to come with some other backbone. How this can be achive.


Murali Krishna


Re: Problem with BGP


you could advertise a partial block in BGP

router bgp 65000

no synchronization

no auto-summary

network mask

network mask


neighbor remote-as 65001

neighbor prefix-list 101 out

neighbor remote-as 65002

neighbor prefix-list 102 out

ip route Null0 250

ip route mask Null0 250

ip route mask Null0 250

ip prefix-list 101 seq 5 deny

ip prefix-list 101 seq 10 permit le 32

ip prefix-list 102 seq 5 deny

ip prefix-list 102 seq 10 permit le 32

This would announce to AS 65001 and to AS 65002. Additionally both ASes get the full address block (plus all other routes) to ensure connectivity for the full address block if one neighbor goes down.

Be aware however, that in the internet it is highly unlikely to get updates announced smaller than /24. Most large ISPs will block anything smaller than /24.

Hope this helps! please rate all posts.

Regards, Martin

Community Member

Re: Problem with BGP

Thanx Martin i will configure and i will test that

Community Member

Re: Problem with BGP

Thx Martin for the crisp explaination

Community Member

Re: Problem with BGP

Hello Murali,

Have tried using med.

1.Tag a higher med to all the routes which you do not want to be reached from one backbone and announce it.

You can follow this procedure to all backbone routers and influence backbone to follow what you want. Again it depends on what attributes your ISP is checking or considering on their end.

You can use route-map to achieve this.


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