Computer Certification

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
/24. They're also connected by a BRI ISDN link
172.12
/24. R1 is advertising a loopback network
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

1.0.0.0/32 is subnetted
1
subnets

O 1.1.1.1 [110/65] via 172.12.123.1
0
Serial0


R2#ping 1.1.1.1

Type escape sequence to abort.

Sending 5
100
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 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


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
0
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
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 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
0
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

A floating static route is an excellent "back door" that will keep the ISDN link down while allowing that link to serve as a backup route. Just make sure the ISDN link comes down when you expect it to - always check that with show isdn status!