|
|
|
|
|
|
|
|
|
|
Cisco CCNA / CCNP Certification Exam Tutorial: Floating Static RoutesKnowing 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, 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
Type escape sequence to abort.
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 ?
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
Let's see the effect on the routing table when the Serial0 interface is closed. R2(config)#int s0
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.
12:16:01: BR0 DDR: Dialing cause ip (s=172.12.12.2, d=1.1.1.1)
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#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface
Article Tags: 1111 110/65, Rout 1000/32 Source: Free Articles from ArticlesFactory.com
ABOUT THE AUTHORChris Bryant, CCIE #12933, is the owner of The Bryant Advantage (http://www.thebryantadvantage.com). For a copy of his FREE "How To Pass The CCNA" or "CCNP" ebook, visit the website and download your copies! Daily exam questions and tutorials now available through RSS feed!
|
|||||||||||||||||||||||||||||||||||||||||
Partners
|
