The routes we 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 contains 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 route types.
| Route Source | Default AD |
| Connected interface | 0 |
| Static route | 1 |
| External BGP | 20 |
| EIGRP | 90 |
| OSPF | 110 |
| RIP | 120 |
| External EIGRP | 170 |
| Internal BGP | 200 |
| Unknown | 255 |
Routers select the route that has a lower AD value. For example, the routing table has 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 the route added by any 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 the static route as a backup route in a network that uses routing protocols to add routing information to routing tables.
This tutorial is part of the tutorial series "Understanding route types in the routing table.". Other parts of this series are the following.
Chapter 1 The show IP route command on Routers
Chapter 2 Routing table updating methods
Chapter 3 Directly connected routes and Local routes
Chapter 4 Static and Floating Static Routes Explained
Chapter 5 Static and Dynamic routes in the routing table
Chapter 6 Default Route in the Routing Table
Chapter 7 Network Routes and Host Routes Explained
Let us take an example.
The following image shows a network.
The administrator configured the RIP routing protocol on this network. The RIP routing protocol discovered all network routes and added them to the routing table of all routers.
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.
The administrator adds a static route via 192.168.0.10 to reach the network 30.0.0.0/8. The administrator wants R2 to take this route when route 192.168.0.5 is down.
But 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 will replace the existing route derived from RIP. If multiple routes for the same destination exist, the router keeps only the route having the lowest AD value in the routing table. The AD value of the static route (1) is lower than the AD value of RIP (120).
The administrator can solve this issue by changing the AD value of the first route. He can change it to any value higher than the AD value of the route added by the RIP protocol. For example, he can change it to 125.
After this, the route will take the first route to reach R3 only when the second route is not available.
When we change the default AD value of a static route, the static route 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.
