In order to have a fully fault tolerant network, a backup solution for WAN links is vital. There are many options for configuring a backup line incase the primary line fails and in this tutorial we are going to look at using floating static routes to achieve a dial-up connection to act as our backup for our primary serial line.

Using floating static routing as a backup solution works on the following principle:

1. A dynamic routing protocol is running over your primary line
2. When the link fails, the routing updates will fail and the routing table will flush
3. A static route that uses the dial-up interface will become the best route
4. The backup interface will dial and traffic will continue to flow
5. When the primary line comes back up the dynamic routing protocol will fill the routing table, overwriting the floating static.

 

NOTE: Any type of dial-up interface may be used (modem / ISDN / 3G etc)

 

Based on the above it’s important to understand a few things about routing.

• A router will look for a route with the longest match (most specific route wins).
• If more than one identical route exists, the route with the LOWEST administration distance will be inserted into the routing table

 

So in order for us to configure floating static routes, we need 2 things configured.

• A dynamic routing protocol MUST be configured for this solution
• An identical static route must be created with an administrative distance higher than our routing protocol.

 

Example:

If I am running EIGRP as my routing protocol and it has learnt a route to network 192.168.1.0 /24 with an admin distance of 90 ( EIGRP has an administrative distance of 90 by default). I must create a static route for the network 192.168.1.0 /24 with a next hop of my remote routers dial-up interface. A static route however has a default administrative distance of 1, which at this point would mean it would overwrite my dynamically learnt route and all my traffic would be sent over my dial-up interface leaving me with a rather large phone bill. So when I create my static route it’s imperative that I change the default administrative distance to something higher than that of my routing protocol – I suggest a value of 250.

My static route would look like this:

AOIP.ORG(config)# ip route 192.168.1.0 255.255.255.0 10.0.1.1 200

Destination network : 192.168.1.0

Subnetmask for destination : 255.255.255.0

Next hop address of the remote routers dial-up interface: 10.0.1.1

Administrative Distance : 200