To pass the Cisco CCNA and CCNP certification exams, in addition to turning into a world-class networker, you've got to understand how and when to make use of floating static routes. And if you're questioning what makes them "float" -- read on!
On this instance, R1 and R2 are running OSPF over a Frame Relay network, 172.12.123.zero /24. They're also connected by a BRI ISDN hyperlink, 172.12.12.0 /24. R1 is promoting a loopback community, 1.1.1.1 /32, by way of OSPF. We wish R2 to have a path to that loopback even if the body goes down - and right here, we'll use a floating static path to make that happen.
R2 sees the path to the loopback interface through OSPF, and may ping that interface successfully.
R2show ip route ospf
1.0.0.zero/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:02, Serial0
R2ping 1.1.1.1
Sort escape sequence to abort.
Sending 5, a hundred-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
!!!!!
Success fee is one hundred pc (5/5), round-trip min/avg/max = 68/68/68 ms
This is when it is important to know your administrative distances.... or at the very least know the place to look to see them! The AD of OSPF is a hundred and ten, 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 larger than a hundred and ten, it will not be used until the OSPF route leaves the routing table. That's why this kind of route is known as a "floating" static route - the route "floats" in the routing table and is not seen except the primary route leaves the table.
You discovered the way to write a static route in your CCNA research, but you additionally do not forget that the default AD of a static route is either 1 or 0... and each of those values are less than 110! To vary the AD of a static route, configure the desired distance at the end of the ip route command.
The static route has an AD that is just one higher than that of the OSPF route, but that is sufficient to make the route "float" and never yet be seen in the routing table.
R2show ip route
1.0.0.zero/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] through 172.12.123.1, 00:06:44, Serial0
172.12.0.zero/24 is subnetted, 2 subnets
C 172.12.12.zero is directly linked, BRI0
C 172.12.123.0 is instantly related, Serial0
Let's examine the impact 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, modified state to administratively down
12:04:fifty six: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0, modified state to down
R2show ip route
1.0.0.zero/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.zero is immediately related, BRI0
The floating static route seems in the desk, however the ISDN hyperlink won't come up till 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.
R2ping 1.1.1.1
Kind escape sequence to abort.
Sending 5, a hundred-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
12:16:01: BR0 DDR: Dialing trigger ip (s=172.12.12.2, d=1.1.1.1)
12:16:01: BR0 DDR: Trying to dial 8358661
12:16:01: %LINK-3-UPDOWN: Interface BRI0:1, modified state to up.!!
12:16:01: BR0:1 DDR: dialer protocol up!!
Success fee is eighty p.c (four/5), round-journey min/avg/max = 36/37/forty ms
The link comes up and site visitors can still attain 1.1.1.1. As soon as R2 turns into an OSPF neighbor of R1 once more, the OSPF route will again develop into the primary path and the floating static route leaves the routing table.
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.zero is instantly linked, BRI0
C 172.12.123.zero is straight related, Serial0
A floating static route is a wonderful "again door" that will hold the ISDN hyperlink down whereas permitting that hyperlink to function a backup route. Just be certain that the ISDN link comes down when you count on it to - all the time test that with present isdn standing
