Cisco CCNA / CCNP Certification Exam Tutorial: Floating Static Routes
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
1.0.0.0/32 is subnetted, 1 subnets
O 1.1.1.1 [110/65] via 172.12.123.1, 00:00:02, 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 (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, 00:06:44, 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-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, 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, 00:00:16, 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!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage. He offers free CCNA, CCNP, and Home Lab Setup Tutorials, as well as the world's most comprehensive CCNA Study Guide. Online boot camps, video training, and binary/subnetting help are also available. Clear, concisce, and comprehensive, Chris' CCNA and CCNP training is the finest available. Join the revolution against overpriced and incomplete tech manuals and classes - visit http://www.thebryantadvantage.com/ today!
Your Article Search Directory : Find in ArticlesRecent articles in this category:
- Understanding Cisco Sfp Optical Transceivers
The word SFP is an IT industry term which stands for Small Form Factor Pluggable. An SFP is a variet - Sharing The Internet Connection
If you are connecting two or more of your computers with each other over the network, then it is eas - Connecting The New Computer To My Server
The server is the computer that will handle the requests for applications, data and also other netwo - Networks
What is a network and its types ?The generic term " network "defines a set of entities (objects, peo - Ssl Certificates Are In Use Today Not All Valid
Qualys, a security research firm tries to paint a detailed portrait of SSL deployments and gaps with - A Mail Forwarding Usa - A Gift To Global Trade Links
Technologies have evolved very much over the past few decades. Globalization has enabled the world t - Benefits Of Wan Optimization Appliances/devices
In today's globally competitive market, corporate organizations both big and small adopt strategic m - Proper Network Cable Management And How It Can Save Your Business Time And Money
In many businesses now there are plenty of computers which will likely be attached to some sort of s - Dedicated Hosting In India For Web Servers
Dedicated web server hosting in India for complete control over your dedicated server is provided by - Outsourcing: The Future Of Network Management India
Computer Networking is a dynamic area and few strategies that were not thinkable yesterday can becom
Most viewed articles in this category:
- Image Sticking In Lcd Tv Sets
Image Retention - Isn't it just an issue with Plasma TVs?What is Image Sticking?Many are aware that - Used Foundry Networks: Used IT Gear Goes Mainstream
San Francisco, CA April 10, 2006Ask any IT manager what they think about used data networking gear a - CCNP Certification / BCMSN Exam Tutorial: Server Load Balancing (SLB)
When you're working on your BCMSN exam on your way to CCNP certification, you'll read at length abou - CCNP Certification / BSCI Exam Tutorial: The BGP Neighbor Process
Like TCP, BGP is connection-oriented. An underlying connection between two BGP speakers is est - CCNA / MCSE / CCNP Certification: Making Failure Work For You
Whether you're on the road to the CCNA, CCNP, MCSE, or you're on any other computer certification tr - Cisco CCNP / BSCI Exam Tutorial: OSPF Route Redistribution Review
OSPF route redistribution is an important topic on the BSCI exam, and it's a topic full of details a - Cisco CCNP / BSCI Exam Tutorial: 10 ISIS Details You Must Know!
Earning your CCNP certification and passing the BSCI exam depends on knowing the details of many Cis - Cisco CCNA / CCNP / BCMSN Exam Review: Trunking And Trunking Protocols
To earn your CCNA or CCNP certification, you've got to understand the basics of trunking. This - Cisco CCNA / CCNP Certification Exam Tutorial: ISDN And Multilink PPP
ISDN is a huge topic on both your Cisco CCNA and BCRAN CCNP exams. While many ISDN topics seem - Used foundry networks hardware remains in high demand
San Francisco, CA May 22, 2006: In recent years, the trend towards used networking hardware has cont