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To pass the Cisco CCNA and CCNP certification exams, as well as becoming a world-class networker, you've got to know how and when to use floating static routes. And if you're wondering what makes them "float" -- read on! In this example, R1 and R2 are running OSPF over a Frame Relay network, 172.12.123.0 /24. They're also connected by a BRI ISDN link, 172.12.12.0 /24. R1 is advertising a loopback network, 1.1.1.1 /32, via OSPF. We want R2 to have a route to that loopback even if the frame goes down - and here, we'll use a floating static route to make that happen. R2 sees the route to the loopback interface via OSPF, and can ping that interface successfully. R2#show ip route ospf
Type escape sequence to abort.
This is when it's important to know your administrative distances.... or at least know where to look to see them! The AD of OSPF is 110, which means we can configure a static route to 1.1.1.1 /32, and as long as the AD of the static route is higher than 110, it won't be used unless the OSPF route leaves the routing table. That's why this kind of route is called a "floating" static route - the route "floats" in the routing table and isn't seen unless the primary route leaves the table. You learned how to write a static route in your CCNA studies, but you also remember that the default AD of a static route is either 1 or 0... R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 ?
R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 111 The static route has an AD that's only one higher than that of the OSPF route, but that's enough to make the route "float" and not yet be seen in the routing table. R2#show ip route
Let's see the effect on the routing table when the Serial0 interface is closed. R2(config)#int s0
The floating static route appears in the table, but the ISDN link will not come up until the BRI interface has traffic to send. Let's ping 1.1.1.1 and see what happens. debug dialer was configured on R2 before sending the ping. R2#ping 1.1.1.1 Type escape sequence to abort.
12:16:01: BR0 DDR: Dialing cause ip (s=172.12.12.2, d=1.1.1.1)
The link comes up and traffic can still reach 1.1.1.1. Once R2 becomes an OSPF neighbor of R1 again, the OSPF route will again become the primary path and the floating static route leaves the routing table. R2(config)#int s0
R2#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface
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