Floating Static Route

Floating Static Route is a route with a Higher Administrative Distance than the primary route. This route acts as a backup route in case the primary route is down.

Let's look at the example below. Here you can see a network topology with R1, R2, and R3.

Figure 1 – Floating Static Route Topology

In this topology, R1 wants to reach the 192.168.3.0/24 network.

It can use two paths:

• The primary route goes through R2 using OSPF, which has an administrative distance of 110.

• The backup path uses a Floating Static route with an administrative distance of 130.

Primary Route with OSPF

R1 installs the route with the lowest administrative distance in its routing table. In this case, it will select and use the OSPF route, which has an administrative distance of 110.

R1 can send traffic through R2 and reach 192.168.3.0/24.

Figure 2 – OSPF Preferred Path and Floating Static Route Fallback

Floating Static Route on OSPF Failure

But now let me show you why the Floating Static Route is used. Imagine the link between R1 and R2 is down. The OSPF neighbor adjacency will go down and the route for the 192.168.3.0/24 network learned via OSPF will be lost.

Since the path with R1 -> R3 -> 192.168.3.0/24 does not use OSPF, a floating static route can be used. In case the primary route goes down, a static route with a higher administrative distance can be installed during the outage of the primary route.

Figure 3 – Floating Static Route Activation After OSPF Failure

Here, R1 will install the floating static route in its routing table and will be able to send traffic to 192.168.3.0/24.

Restoring the OSPF Route

When the link between R1 and R2 is back, the router will use the OSPF route again. This happens because the router prefers the route with the lowest administrative distance.

Figure 4 – Reinstating OSPF as the Primary Route After Link Recovery

Why not use OSPF everywhere?

One question you can ask yourself is: "

Okay, but why didn't we use OSPF on all routers? OSPF can decide on a new route."

That's a great question! The answer is simple: you can't always run a dynamic routing protocol everywhere.

R3 might be a client router you can’t control. Since OSPF can’t run on the link between R1 and R3, using a floating static route is the best option.