Backup Default Route over MPLS

[nkillgore]

We currently have about 15 remote sites and 2 data centers. Each remote site has an MPLS connection and a backup internet connection from the local ISP that routes internal traffic over a VPN tunnel and internet traffic straight out through the local connection. The data centers each have very large internet and MPLS connections.

Each branch is configured with a default route to go out the local ISP connection (ip route 0.0.0.0 0.0.0.0 gi0/1). I would like to advertise a default route via BGP so that we are only depending on ISPs as a tertiary backup.

So, my questions:

1. How do I advertise a default route via BGP?
   1. If I do only that, will it override the locally configured default route? I'm thinking no, but just want to make certain.
2. How do I advertise a backup default route via BGP that points to the secondary data center?
3. How do I make the internet link be a backup to the backup default route?

Thanks,

Neill

[mfurnival]

On your DC routers you will need "default-information originate" under the router bgp section - this will tell them to advertise a default route into the MPLS cloud. You can use a route-map on the secondary DC to make the default route advertised from there appear less favourable than the primary.

On your spoke sites you should change your static route to be a floating static route so it will only come into effect if both DC routes are not available.

[Giuseppe Larosa]

Hello Neill,

1) how  do I advertise a default route via BGP ?

neighbor x.x.x.x default-originate

OR

network 0.0.0.0   ( a default route has to be present in the local node IP routing table)

1.2)

If I do only that, will it override the locally configured default route? I'm thinking no, but just want to make certain.

No, the static default route will be preferred for its lower AD

2)

How do I advertise a backup default route via BGP that points to the secondary data center?

You should advertise a default route in BGP from your routers connecting the secondary data center to the MPLS cloud.

As path prepending can be used to make it a tertiary route

3)

As explained above the backup of the backup route may use AS path prepending to make it less attractive then the backup route

Hope to help

Giuseppe

[Jose Jara]

Hi Neil,

You may also redistribute the default:

router bgp xx

redistribute static

default-information originate

One thing I would never do is to configure a default route pointing to an interface, specially if it´s ethernet, because of:

1. It will work only as long as the router directly connected  has proxy-arp enabled.

2. You may have a big issue as for any destination you are sending traffic you will do an ARP Request, with an increase of the CPU and a very big ARP table.

3. If you have other static routes, could resolve the next-hop through the default route even though the next hop is unreachable, creating a potential black hole.

I always prefer to use the interface + next-hop in static routes.

Hope this helps,

Jose.