Static and Floating Static Routes Explained

The routes you manually add to the routing table are known as static routes. Routers use the letter S to indicate static routes in the routing table. A static route uses the default AD value. A floating static route uses the custom AD value. If the routing table contains two or more routes for the same destination, the router compares their AD (Administrative Distance) value to select the best route. An AD value is an integer from 0 to 255, where 0 is the most trusted and 255 is the least trusted route.

The following table lists the AD values of common route types.

Routers select the route with the least AD value. For example, a router learns two routes for a destination. The AD value of the first route is 90. The AD value of the second route is 1. The router will use the second route to reach the destination. Since the AD value of a static route is lower than that of any route added by a routing protocol, the router always uses the static route if it is available to reach the destination. It creates a problem when you want to use a static route as a backup route in a network that uses routing protocols to add routing information to routing tables.

Let us take an example. The following image shows a network.

You configure the RIP routing protocol in this network. The RIP routing protocol discovers all network routes and adds them to the routing table. The following image shows the output of the show ip route command on R2.

RIP added the route 192.168.0.5 to reach the network 30.0.0.0/8.
Now, for backup, suppose you add a static route via 192.168.0.10 to reach the network 30.0.0.0/8.

You want R2 to take this route when route 192.168.0.5 is down. However, the router will not do this. As soon as you add a static route to reach the network 30.0.0.0/8, the router replaces the RIP-derived route. If multiple routes for the same destination exist, the router keeps only the route with the least AD value in the routing table. The AD value of the static route (1) is lower than the AD value of RIP (120).

You can solve this issue by changing the AD value of the static route. You can change it to any value higher than the AD value of the route added by the RIP protocol. For example, you can change it to 125. After that, the router will use the static route to reach R3 only when the RIP-learned route is unavailable.

A static route with a custom AD is called a floating static route. In other words, a static route with the default AD value is a static route. A static route with a custom AD value is a floating static route.

Conclusion

A static route is a manually added route in the routing table. It has a lower AD value than all routing protocols. A router uses the AD value to compare and select the best route for a destination. If a static route is available, it never selects the routes added by routing protocols, as they have higher AD values. You can change the default AD value of a static route to a value higher than the AD value of the configured routing protocol. It will force the router to use the route the routing protocol adds as the primary, and the static route as the secondary or backup. A static route with a custom AD value is called a floating static route.