BGP Prefix withdrawal

Unanswered Question
Oct 27th, 2009

I have 2 sites peering with different ISP. I have advertising my own address space (192.168.254.0). both of my sites connceted via layer2 and have ibgp sesssions between my screen routers (AS 100). Site 1 is the primary site. All connections from the web routed via site 1.

Problem: when the south interface (eth1/1) on router 1 goes down, router 1 is still advertising 192.168.254.0 because router 1 and 2 both have static route for 192.168.254.0 to null 0 (by design - i cannot change it at this stage). What i would like to do it come up with a better solution so that when any of the south interface (eth1/1) is down i want Router 1 in site 1 to withdraw from adverting 192.168.254.0 prefix. I have looked at conditonal avertisement but dont think it will work for me....Need soe sort of interface tracking for bgp..

any ideas

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Giuseppe Larosa Tue, 10/27/2009 - 06:30

Hello Francisco,

I see two possible solutions:

a) use of aggregate-address for generating 192.168.254.0 + a more specific network command in BGP that fails when the link fails.

this can be done easily if the link has an IP subnet taken from the block 192.168.254.0.

OR usage of a routing protocol to provide knowledge of some 192.168.254.0 components to edge router again this should be able to remove the aggregate when the link fails.

b) if a dynamic IGP cannot be used on the link for security reasons or othe policies.

you can try to take advantage of reliable static routing with object tracking.

the idea is that if the static route is removed the corresponding prefix will be withdrwan as needed.

see

the change is that you don't configure the backup static route but only the primary "monitored" static route.

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html

Hope to help

Giuseppe

francisco_1 Tue, 10/27/2009 - 06:36

does the reliable static routing with object tracking works with bgp?

I am just trying to work out what is the best way to implement it!

Giuseppe Larosa Tue, 10/27/2009 - 07:32

Hello Francisco,

if tracking a static route to null0 works you should be able to do this.

in the track you need to call an object closely related to the south link, that is it can be an IP SLA making an icmp test towards ip address of device at the other end of south link.

if the static route is correctly removed from IP routing table when track test fails the network should be removed from BGP table too.

I would start by verifying the capability to remove the static route when track fails.

(in theory of course it should work)

Hope to help

Giuseppe

francisco_1 Tue, 10/27/2009 - 07:40

Giuseppe,

thanks.

I will test this in the lab and come back to you..

Francisco

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