BGP Conditional Advertisement

Unanswered Question
Nov 24th, 2008

I know we can conditionally not advertise routes if a certain route/s are not in the BGP table, but I'm not sure how to stop advertising the entire BGP table (if we are providing a full routing table to a customer) if a route or routes are missing from the BGP table. Question is, is there a way I can stop advertising a full BGP table to a customer if some of those routes are missing?

I have this problem too.
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Giuseppe Larosa Mon, 11/24/2008 - 12:47

Hello David,

BGP conditional advertisement has added also the exist-map :

you should be able to implement the desired logic : until a prefix described in the route map invoked with the exist-map option you can advertise all prefixes.

With Cisco IOS® 12.2T or later versions, the exist-map keyword can be used to accomplish these tasks as well:

*

If 192.168.50.0/24 exists in R102's BGP table, then advertise the 128.16.16.0/24 network to R101.

*

If 192.168.50.0/24 does not exist in R102's BGP table, then do not advertise the 128.16.16.0/24 network to R101.

see

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml#ex3

Hope to help

Giuseppe

marikakis Tue, 11/25/2008 - 01:54

Hello,

I think the scenario described in this conversation is a tricky one. The documentation for neighbor advertise-map command says: "To install a Border Gateway Protocol (BGP) route as a locally originated route in the BGP routing table for conditional advertisement, use the neighbor advertise-map command in router configuration mode."

The word "advertise" is commonly used loosely to mean either locally originated routes or routes that are actually passed from one AS to another. However, it seems to me that the documentation means locally originated routes in the case of advertise-map. In addition, the routes covered by an advertise-map are also covered by a network command in the examples I've seen.

So, either documentation is not accurate and conditional advertisement works in this scenario without issues or documentation is accurate and you would be trying to locally originate the full routing table (this will probably fail due to no route in IGP or lack of network/redistribute command).

I do not know of another feature that might be of help. The normal filtering with route-maps on actual sessions cannot accomplish the desired effect because this type of route-map is somewhat memoryless (acts on one route at a time following non-deterministic order and disregarding previously considered routes or future ones).

Kind Regards,

M.

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