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Cisco CCNA CCNP Certification Exam Tutorial: Floating Static Routes Computers Articles | July 12 Matthew Bazarevitsch Sun Devils Jersey , 2006

Knowing when and how to configure floating static routes is an important skill on your Cisco certification exams, as well as working in real-world networks. Learn all about floating static routes from Chris Bryant, CCIE #12933.

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 Khaylan Thomas Sun Devils Jersey , 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


1.0.0.032 is subnetted, 1 subnets


O 1.1.1.1 [11065] via 172.12.123.1, 00:00:02 Isaiah Floyd Sun Devils Jersey , 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 (55), round-trip minavgmax = 686868 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 Frank Darby Sun Devils Jersey , 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


1.0.0.032 is subnetted, 1 subnets


O 1.1.1.1 [11065] via 172.12.123.1, 00:06:44, Serial0


172.12.0.024 is subnetted, 2 subnets


C 172.12.12.0 is directly connected Casey Tucker Sun Devils Jersey , 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


1.0.0.032 is subnetted, 1 subnets


S 1.1.1.1 is directly connected A.J. Carter Sun Devils Jersey , BRI0


172.12.0.024 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 Ty Thomas College Jersey , changed state to up.!!


12:16:01: BR0:1 DDR: dialer protocol up!!


Success rate is 80 percent (45), round-trip minavgmax = 363740 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 FULLDR 00:01:57 172.12.123.1 Serial0


R2#show ip route


1.0.0.032 is subnetted, 1 subnets


O 1.1.1.1 [11065] via 172.12.123.1, 00:00:16, Serial0


172.12.0.024 is subnetted, 2 subnets


C 172.12.12.0 is directly connected Shannon Forman College Jersey , 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! Article Tags: Floating Static Routes, Floating Static Route, Directly Connected Bri0, Cisco Ccna, Ccnp Certification, Floating Static Ryan Kelley College Jersey , Static Routes, Isdn Link, Static Route, 1111 11065, Ospf Route, Route Leaves,

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