Static floating route is static route like any other but with added administrative distance in the configuration
R1(config)#ip route 172.16.10.0 255.255.255.0 10.10.10.2 200
Defining the packets route using Static Floating Routes is very interesting topic so I decided to give you a short description of Static floating routes with an example. Static floating route is the same as normal static route except that this kind of static route has Administrative distance configured to some other value than 1.
Remember that if we configure normal static route like this:
R1(config)#ip route 172.16.10.0 255.255.255.0 10.10.10.2
It will send all packets destined for 172.16.10.0/24 network to the neighbor with interface address 10.10.10.2 and of course that static route will have Administrative distance (AD) of 1 by default. If we make the configuration like this:
R1(config)#ip route 172.16.10.0 255.255.255.0 Serial 0/0
In this case the AD will be zero (0). Pretty cool right? I didn’t know that difference for a long time but there is another article in the process of writing that explains why that is so.
In either cases of course, this would be normal because almost every time we configure the static route to override some routing protocol decision. But what if we want to use a static route to make something completely opposite? If we want to use static route only to be a backup route to some subnet then we will need to give the precedence to the path learned by some IGP (Interior Gateway Protocol) like OSPF for example. We know that most paths (routes) learned by OSPF protocol have Administrative Distance of 110. In that case the Administrative distance of some static route needs to be bigger than 110 if we don’t want to kick out the OSPF route from routing table.
If we configure the route like this:
R1(config)#ip route 172.16.10.0 255.255.255.0 10.10.20.2 200
If you see this configuration above, you will notice that after next-hop address 10.10.10.2 there is number 200 added. This part of the command will override the default Administrative distance of the route 1 and configure the same static route with Administrative distance of 200. That’s why this static route will not enter the routing table until there is OSPF path to the subnet 172.16.10.0/24. If for some reason the router R1 loses the prefix 172.16.10.0 learned by OSPF it will then use the static FLOATING route configured above.
From there is the name static floating? Yes! This kind of static route is able to enter and exit the routing table base on other routes to the same subnet learned from some IGP in the process.
So that is the main difference between „normal“ static route and floating one. Normal static route is permanently entered in the routing table and the other one is not.
I made a simple example to show how this works. Let’s say that R1 is a branch router that will use a floating static route to send Enterprise traffic across the Internet but only if the leased line is broken or when the IGP-learned path across the private link fails. This is a normal situation where some branch office is connected to the company infrastructure with private link and uses Internet connection for the backup.
In this example R1 is using OSPF over the leased line (connection to R2 S0/0 – S0/0) and there is a configuration of a floating static route to 172.16.10.0/24 that routes traffic over the Internet connection (S0/1 – S0/1). Router R1 would prefer an OSPF learned, AD 110 route for 172.16.10.0/24 instead, until that route through S0/0 fails.
Basically if there would be any communication break on the leased line, the static floating route would direct that traffic across the Internet. In that way the communication will not experience any outage and the leased line will be used at his best using IGP protocol when online.