Cisco CCNP Certification Exam Training:

Floating Static Routes

By Chris Bryant, CCIE #12933

Cisco technologies get more complex as you work on your CCNP certification, but sometimes the simplest solutions are the best.

Under certain circumstances, a simple static route may be all the backup solution you need.  In the following exhibit, R1 and R2 are running OSPF over the Frame network.  R1 is also advertising its loopback interface 1.1.1.1 /32 into the OSPF domain as well.

R2 sees the route to the loopback interface and can ping that interface successfully.

R2#show ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/65] via 172.12.123.1, 00:00:02, Serial0

R2#ping 1.1.1.1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 68/68/68 ms

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-derived 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... and both of those values are less than 110!   To change the AD of a static route, configure the desired distance at the end of the ip route command.

R2(config)#ip route 1.1.1.1 255.255.255.255 bri0 ?
  <1-255>    Distance metric for this route
  A.B.C.D    Forwarding router's address
  name       Specify name of the next hop
  permanent  permanent route
  tag        Set tag for this route
  <cr>

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
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/65] via 172.12.123.1, 00:06:44, Serial0
     172.12.0.0/24 is subnetted, 2 subnets
C       172.12.12.0 is directly connected, BRI0
C       172.12.123.0 is directly connected, Serial0

Let's see the effect on the routing table when the Serial0 interface is closed.

R2(config)#int s0
R2(config-if)#shutdown

12:04:53: %OSPF-5-ADJCHG: Process 1, Nbr 172.12.123.1 on Serial0 from FULL to DOWN, Neighbor Down: Interface down or detached

12:04:55: %SYS-5-CONFIG_I: Configured from console by console
12:04:55: %LINK-5-CHANGED: Interface Serial0, changed state to administratively down

12:04:56: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, changed state to down

R2#show ip route
     1.0.0.0/32 is subnetted, 1 subnets
S       1.1.1.1 is directly connected, BRI0
     172.12.0.0/24 is subnetted, 1 subnets
C       172.12.12.0 is directly connected, BRI0

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.
Sending 5, 100-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:

12:16:01: BR0 DDR: Dialing cause ip (s=172.12.12.2, d=1.1.1.1)
12:16:01: BR0 DDR: Attempting to dial 8358661
12:16:01: %LINK-3-UPDOWN: Interface BRI0:1, changed state to up.!!
12:16:01: BR0:1 DDR: dialer protocol up!!
Success rate is 80 percent (4/5), round-trip min/avg/max = 36/37/40 ms

The link comes up and traffic can still reach 1.1.1.1.  Once R2 becomes an OSPF neighbor of R1 again, the OSPF-derived route will again become the primary path and the floating static route leaves the routing table.

R2(config)#int s0
R2(config-if)#no shut

R2#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
172.12.123.1      1   FULL/DR         00:01:57    172.12.123.1    Serial0

R2#show ip route
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/65] via 172.12.123.1, 00:00:16, Serial0
     172.12.0.0/24 is subnetted, 2 subnets
C       172.12.12.0 is directly connected, BRI0
C       172.12.123.0 is directly connected, Serial0

Simply by adjusting the administrative distance of a static route, we've got a backup route that takes little configuration. Floating static routes are an important step in your CCNP certification training and working in production networks, so make sure you can write one when needed!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com