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BGP Network Advertising

Posted on 2014-04-02
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Last Modified: 2016-11-23
I have network behind core switch is 192.168.10.0/25
and my routing to corporate network is BGP routing. I published my network at BGP as 192.168.10.0/24 because we have other subnet that also using 192.168.10.241/28 that doing routing internally.

at Corp router, they only allowed our subnet advertise as /24.
all my server which is using /25 subnet is not able to reach the next hop at Corp. which according to them they can't find my subnet advertised at their route table.
if to ask them change to accept my subnet which is /25 required a lot of process and paper work and question that to answer to justify it. the flow may take longer.

i understand that auto summarization may help... but i seem can't find the command in my switch.

By the way, I'm using Dell Force10 S4810. all this works perfectly until the host subnet been requested to change to /25

tks
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Question by:hell_angel
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pergr earned 500 total points
ID: 39974135
Most likely the issue is that in order to advertise a /24 route in BGP, you first must have that /24 route in your routing table.

You typically do that by setting a static route for the /24, with next-hop as "null0" or "discard", or what ever the vendor software will call it.

Once that is done, the /24 will be advertised and the Corp should see it.
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by:hell_angel
ID: 39974234
it solved.

added ip route to null and aggregate -address
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LVL 28

Expert Comment

by:mikebernhardt
ID: 39975745
For your router to advertise the BGP route of 192.168.10.0/24, you need to have that route in your internal routing table already. What you want to do is correct though.

The solution is to put a static route in your BGP router:
ip route 192.168.10.0 255.255.255.0 null0

This will allow your router to advertise that route in BGP. Because routers always route to the most specific route, your existing routes will continue to work. Any traffic sent to a /24 destination that doesn't actually exist will simply be dropped.
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