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BGP Multihomed

We have a multihoming environment with 2 SP.

The border routers are running HSRP protocol.

How could we know, when one of the SP has a problem if the peer bgp are OK and the router interface is up ?

Cisco Employee

Re: BGP Multihomed

You could use a recursive default route to a prefix that is part of the Internet infrastructure. This way, your default route would be resolved only if the specific prefix is present. This would ensure that the SP is functioning normally.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
México móvil: +52 1 55 8312 4915
Cisco México 
Paseo de la Reforma 222 Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600

Re: BGP Multihomed

HSRP has nothing to do with the problem. The solution lies in BGP.

If you are running defaultless, the failing SP _should_ only send you prefixes which they can still reach, so you don't have to do anything special.

If you are currently accepting a default route from either ISP, modifiy your configuration to make the default route conditional on the ISP peer router having a valid route to one of several prefixes upstream of the ISP. That way, if they loose connectivity through their network, you will stop sending default traffic to them.

The conditional default route can be configured either on your router (in which case, you need to negotiate with the ISP to advertise the trigger routes to you) or on the ISP's router (so they can change the trigger prefix to optimally reflect the status of their network as their topology and connectivity change over time).

At worst, you can tell your service provider you want all routes and throw away all but the few you care about at your end. The disadvantage of this approach is that your line will be consumed downloading a few hundred thousand useless routes, delaying your ability to fill it up with traffic.

If you need more info, there are examples of both approaches discussed in Chapter 8 of my book, and example configurations on my web site (see listings 8-22, 8-23 and 8-24).

Good luck and have fun!

Vincent C Jones

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