Advertising a default route in BGP with an alternate default route as backup.

So you have your MPLS WAN and your filtering internet at your main data centre, everything going along nicely until the CE router at your data centre goes hard down, all your users are suddenly wondering why they can’t use Facebook or YouTube anymore!
Advertising a default route is easy but advertising multiple…………… now thats a different story.

You need to find a way of making the backup route less desirable, that way it would only be used if the original was unavailable, so here’s how to do it.

You will at least be familiar with BGP path selection, prefix lists, route-maps

Advertise your default route from your main data centre. Here is a sample config

router bgp xxxxx
neighbor x.x.x.x remote-as xxxxx
network 0.0.0.0

In order for BGP to advertise any route it must exist in the routing table so either you use a static route or you’re running a dynamic routing protocol to advertise into your router like EIGRP from your main data centre switch. (I would recommend you use a dynamic routing protocol)
Now that your default route is being advertised this will filter out to the rest of your WAN routers.
So your secondary default route is a little different you still need to advertise this but with some sort of distinguishing feature, the simplest way to do this is to use AS-Prepend this adds on the AS (Autonomous System) number you specify to the advertised route, If you know how BGP Path selection works you will know that BGP will prefer the shortest AS_PATH, regardless of bandwidth or connection type so imagine RIP routing it uses a hop count for route selection it’s the same idea as that. Note. BGP path selection does not solely rely on AS_PATH but for the purposes of this discussion we will assume you are not using WEIGHT, LOCAL_PREF or IGP redistribution.

Firstly you need to create a prefix list to match only the default route

ip prefix-list 10 description Secondary-default
ip prefix-list 10 seq 5 permit 0.0.0.0/0

Next thing you want to do is use a route map to tie in the conditions you need to set for the default route.

route-map default-route permit 5
match ip address prefix-list 10
set as-path prepend xxxxx xxxxx xxxxx xxxxx xxxxx
route-map default-route permit 10

Then all that’s left to do is advertise the route map via BGP

router bgp xxxxx
network 0.0.0.0
neighbor x.x.x.x remote-as xxxxx
neighbor x.x.x.x route-map default-route out

Again making sure the default route exists in your routing table otherwise it won’t be advertised.

Some commands you might use to confirm your changes will be

sh ip bgp
sh ip bgp 0.0.0.0
sh ip bgp neighbor x.x.x.x advertised-routes
sh ip route 0.0.0.0
sh ip route
sh run | inc ip route

Easy as that!

BGP is by far the most versatile and configurable routing protocol I have ever worked with it surprises me every time work with it I learn something new about it.

RH

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