Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

BGP no advertised-routes if a other router is online


I have a backup router to a nother ISP.

But I want only advertised my AS to this ISP if the primary router is

not longer talking to the first ISP, or if my Leased Line to this router is down.

I have 3 router. One is Connected to the ISP1. He get the routes via eigrp from router 2nd. If the Line is down he can not longer advertise the network.

Then I have another router to the ISP2. He also learn the routes via

eigrp from router 2nd. And advertised the learned routes. With as-path prepend 3xtimes my AS. and also with a default-metric 150.

Every things looks good for the backup time.

But I get traffic in over the 3 router.

So my idea is now to stop advertise my AS or the network if Router 1 is reachable. But I have not seen how I can do that.

Do you have a clue for me.




Re: BGP no advertised-routes if a other router is online

You don't want to turn off BGP advertisements on the backup router based on the primary route. Doing so will cause both paths to flap if you have a problem with the primary route and could get your backup route dampened, unnecessarily cutting you off from the internet for an extended period.

The preferred approach for a "emergency use only" advertisement is to use a community string which indicates that this is a backup path and should only be used no other path is available, regardless of AS path length. The value of this community string will depend upon the ISP, and may or may not be a supported feature of your backup ISP.

Alternatively, add more prepends to your advertisement, particularly if your primary path is not a tier one provider. You may find that five or ten prepends are required to make your backup path long enough to be ignored. See Halabi's book "Internet Routing Architectures" for all the gory details.

Good luck and have fun!

Vincent C Jones

New Member

Re: BGP no advertised-routes if a other router is online


BGP conditional advertising will help you.

See more details in BGP documentation.

CreatePlease login to create content