Floating Static Route

Posted on August 26, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |


CCENT may test your understanding about floating static route:

A floating static route is a static route with an administrative distance higher than that of the routing protocol in use. In this way, the floating static route will only appear in the routing table if the dynamically learned route is lost.

Let’s use the following routing table for an example.

R1#show ip route rip
     172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
R       172.12.23.0/27 [120/1] via 172.12.123.2, 00:00:04, Serial0  

                       [120/1] via 172.12.123.3, 00:00:04, Serial0 

Let’s say we want to create a static route to 172.12.23.0 /27 with a next-hop IP address of 210.1.1.3, but it will only be used if these two dynamically learned routes are removed from the table.

If we create a regular static route, the AD of the static route will be 1, and that’s much lower than the AD of these RIP routes!

Therefore, the static route would be placed into the table immediately and the RIP routes would be removed.

To create a floating static route, we need to use the distance option at the end of the ip route command. We really just need to set an AD of 121 for the static route, but in this case we’ll set one of 200.

R1(config)#ip route 172.12.23.0 255.255.255.224 210.1.1.3 ?
  <1-255>    Distance metric for this route
  name       Specify name of the next hop
  permanent  permanent route
  tag        Set tag for this route
  <cr>

R1(config)#ip route 172.12.23.0 255.255.255.224 210.1.1.3  200

Using the option to change the static route’s administrative distance (that’s what “distance metric for this route” refers to) creates the static route, but it will not appear in the routing table unless the matching RIP routes leave the table.  Right now, the static route is not in the table:

R1#show ip route
< code table removed for clarity >

     172.12.0.0/16 is variably subnetted, 3 subnets, 2 masks
C       172.12.13.0/24 is directly connected, Serial1
R       172.12.23.0/27 [120/1] via 172.12.123.2, 00:00:21, Serial0
                       [120/1] via 172.12.123.3, 00:00:05, Serial0
                  
C       172.12.123.0/24 is directly connected, Serial0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, Ethernet0

Serial0 will now be closed.

R1(config)#int s0
R1(config-if)#shutdown

As a result, the RIP routes are lost – note the exit interface for the RIP routes is the interface we just closed.  As a result, the floating static route is now in the table.

Note the administrative distance in the brackets is now 200 and the next-hop IP address for that route has changed. A quick ping shows that we still have connectivity to the remote network.

R1#show ip route
< code table removed for clarity >

     172.12.0.0/27 is subnetted, 1 subnets
S       172.12.23.0 [200/0] via 210.1.1.3
     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, Ethernet0
C    210.1.1.0/24 is directly connected, Serial1

R1#ping 172.12.23.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.12.23.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 36/36/36 ms

When we reopen R1′s S0 interface, the RIP updates will again be received by R1 and the floating static route will be removed from the table due to its higher AD.

R1(config)#int s0
R1(config-if)#no shutdown

R1#show ip route
< code table removed for clarity >

     172.12.0.0/16 is variably subnetted, 2 subnets, 2 masks
R       172.12.23.0/27 [120/1] via 172.12.123.2, 00:00:17, Serial0
                       [120/1] via 172.12.123.3, 00:00:00, Serial0
C       172.12.123.0/24 is directly connected, Serial0
     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.1.0 is directly connected, Ethernet0
C    210.1.1.0/24 is directly connected, Serial1

And that’s a floating static route!

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2 Responses to “Floating Static Route”

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Excellent site, keep up the good work

why dont you put porn/sex positions documentry?


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