One of the more unpleasant experiences for any network engineer is handling Denial of Service attack. Even worse one is dealing with Distributed Denial of Service Attack, which usually looks as if the entire Internet is intent on crashing part of your network. In this article, I will explore one of the possible ways to handle this kind of attack in both enterprise and service provider networks.
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One of the interesting restrictions of BGP is that neighbor will not establish BGP session with a peer if the only way to reach it is through the default route. Is there anything we can do in that scenario?
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Per popular request, this post has the purpose to explain the solution of the problem of BGP peering with an an unknown AS. I would like to start by stating the obvious.

This is a solution in a search of a problem. Popular as it may be with CCIE workbook vendors and rumor has it with CCIE exam authors, real life has almost no use of this. When faced with a log message that you will read about in few paragraphs, solution is using the phone and not doing the other stuff I’m about to write. That said, it’s an interesting and fun problem.

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driymcg3h2

BGP Outbound Route Filtering

R1 and R2 are EBGP Neighbors. R1 is receiving the following routes from R2:

R1#sho ip bgp | be Net
Network          Next Hop            Metric LocPrf Weight Path
*> 200.0.0.0        192.168.12.2             0             0 200 i
*> 200.0.1.0        192.168.12.2             0             0 200 i
*> 200.0.2.0        192.168.12.2             0             0 200 i
*> 200.0.3.0        192.168.12.2             0             0 200 i Read Full Entry  »

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The table-map command is a lesser known and sometimes misunderstood BGP configuration command that can be used to set metrics or route-tags on routes. Here we use it to set a route-tag and verify that the tag stays after redistribution into OSPF.

R2 (AS #2) and R3 (AS #3) are BGP Peers. R3 and R4 are OSPF neighbors. R2 is advertising 192.168.12.0 and R3 is redistributing it into OSPF.

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