dual homed bgp route perference help

[Thomas Park]

Good Morning, sir

I have a question regarding bgp route preference. 

We have multi-homed BGP on remote site. 

Let's call this site , site_a has verizon and at&t MPLS connection using bgp. 

Currently, at&t route is preferred using as path prepend method. 

We have addition AS number prepended on link to Verizon to prefer at&t route. 

At this time, verizon is idle and not being used. 

We would like to utilize this network to pass backup traffic. 

We have source and destination address that we would like to pass through Verizon (out backup) link. 

Any idea how I can do this? 

I created the source/destination with ACL and created route-map matching that ACL and tweaked the local preference. 

and since only major route is listed under 'show ip bgp' and not the specific route which I would like to influence, 

I didn't see the tweaked route on routing table. 

Any idea, anyone? 

Thanks

[Jon Marshall]

You need to get the specific route into the routing table.

How you do that depends on what your existing setup is.

However once you do that if the specific subnets are part of major networks being advertised and preferred over AT&T do you need local preference ie.

at each site you advertise the more specific route on the Verizon link but not the AT&T link and the more specific route will always be used for those subnets.

If the link fails then you still have the major network being advertised on the other link that can be used.

Jon

[Jon Marshall · Accepted solution]

Getting the specific route into the routing depends on what is there ie. -

if there is already a route with same prefix and subnet mask in your IP routing table then you are fine.

If there isn't and your routing table does not have even more specific routes covering the whole subnet you want to advertise then you could use a static route either pointing to the next hop IP or the interface on the router used to get to that subnet.

Then you can use a "network ..." statement under your BGP configuration to advertise it and use a route map to only advertise it down the backup link.

Jon