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David Gibbons
Beginner

BGP Less Specific Advertisements

Hi All,

Looking for a little advise on BGP advertisements. I have two ISPs and several prefixes I'm advertising. One of the prefixes is a /22 and I'd like to advertise one of the /24's inside of that /22 out one provider and the entire /22 out both providers. I am using a route-map for the advertisement and that works.

Current Config:

router bgp XX

  network 1.1.1.0 mask 255.255.252.0

Proposed Config:

router bgp XX

  network 1.1.1.0 mask 255.255.252.0

   network 1.1.1.0 mask 255.255.255.0

What Happens When I Type The Proposed Config:

router bgp XX

  network 1.1.1.0 mask 255.255.252.0

  network 1.1.1.0

So basically I want to advertise the more specific AND the less specific routes. When I add the second smaller network (the /24) that overlaps the /22, the mask is dropped off of the BGP config. Any ideas why?

Thanks,
Dave


4 REPLIES 4
cadet alain
Advisor

Hi,

you want to advertise a /22 summary to one ISP and the summary plus one component /24 subnet to the other ISP?

Why aren't you using the aggregate-address along with suppress-map and unsuppress-map ?

Concerning your BGP config is it reflected in the BGP table: sh ip bgp regexp ^$

also can you post complete BGP config: sh run | s router bgp

Regards.

Alain

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milan.kulik
Advocate

Hi,

I believe that's because the network you are talking about is a  C-class IP network.

And the config is simply not diplaying default subnet masks.

HTH,

Milan

@milan, thank you -- that answers my question perfectly.

Much appreciated, you folks are awesome!

Cheers,

Dave

M-Hassanien
Beginner

Dear David,

BGP will not advertise a prefix unless it is already in the routing table. Do you have both /22 & /24 prefixes in you routing table? can you provide me with the output of :

1) show ip route

2) show ip bgp neighbors X.X.X.X advertised-routes