routing to internet 2 different isp's, conditional routing

[carl_townshend]

hi all

I have seen a setup where we have 2 internet routers, an 10 meg and a 2 meg, these come in on 2 different isp's and we advertise the same routes to the isp's from both routers using bgp, we then have ospf redistributing these routes into our other internet intermediatry routers.

the engineer said there is a conditional statement on the backup router that looks if the route through the primary is there, if it goes the traffic goes to the backup isp,

can anyone tell me how this is achieved ?

cheers

Carl

[John Blakley]

Carl,

In BGP, there's a conditional advertisement statement that uses 2 route-maps. One of the route-maps looks for a condition like if the route is there or not, and the other route map is for what you advertise. For example, if you have an ibgp connection with your primary and you have the route 192.168.1.0/24 in your bgp table, then you could have a route-map stating that if that route goes away, then you need to stop advertising the 192.168.2.0/24 subnet to the provider because you know that your router is the only entry point to the 192.168.2.0/24 subnet. (Assuming the other router doesn't advertise for or pass it. It looks something like this:

ip prefix-list Only2 permit 192.168.2.0/24

ip prefix-list If192 permit 192.168.1.0/24

route-map Advertise permit 10

match ip address prefix Only2

route-map Exist permit 10

match ip address prefix If192

router bgp 10

neighbor advertise-map Advertise exist-map Exist

So, if 192.168.1.0 is in my BGP table then I will advertise 192.168.2.0. If the 192.168.1.0 goes away, then I'll not advertise the 192.168.2.0 to ISP2.

HTH,

John

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