Cisco CCNA / CCNP Certification Exam Tutorial: Floating Static Routes

By: Chris Bryant
Submitted: 2007-01-17 15:44:36
Print this article | Tell a friend | For publisher | Social Bookmarking
Rating:
 

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, home of free CCNA and CCNP tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.

You can also join his RSS feed and visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA / CCNP exam questions! Details are on the website.

For a FREE copy of his latest e-books, “How To Pass The CCNA” and “How To Pass The CCNP”, visit the website and download your free copies. You can also get FREE CCNA and CCNP exam questions every day! And coming in 2007 -- Microsoft Vista certification from The Bryant Advantage!

Article source: Expert Articles

Most Recent Articles in Certification Tests category

  • C Programming Career Courses - UK Explained - By: Jason Kendall
    When choosing any computer training program it is vital that the qualification it leads to falls in line with commercial requirements. In addition, ensure that your training suits you, your personality and abilities. Whether you're hoping to be a whiz with office user skills, or would like to achieve professional qualifications in IT, there are easy to follow courses and support to give you the chance you've been looking for.
  • Networking Training Providers - News - By: Jason Kendall
    These days, many workplaces could not function efficiently if it weren't for support workers fixing PC's and networks, while giving advice to users each and every day. Our country's need for larger numbers of skilled and qualified individuals is growing, as we turn out to be consistently more dependent upon computers in today's environment.
  • Computer Support Training - Options - By: Jason Kendall
    A very small number of men and women in this country are claiming to be happy in their job. The vast majority of course won't do a thing. The fact that you've got this far if nothing else suggests that you know it's time to make a change.
  • Choosing an MCSA in the UK Examined - By: Jason Kendall
    Whether you're just about to get started, or a professional ready to gain acknowledged certifications, there are interactive MCSA (Microsoft Certified Systems Administrator) study programs that teach both entry levels. For a newcomer to the world of computers, it's likely to be vital to have some coaching prior to getting into your four Microsoft Certified Professional exams (MCP's) that are required to pass the MCSA. Find a training provider that can tailor your studying to fit your requirements - one who will guide you to guarantee that you make the right choices.
  • Computer Training in Cisco CCNA Clarified - By: Jason Kendall
    Training in Cisco is fundamentally for those who wish to understand and work with routers and network switches. Routers join up computer networks over the internet or lines dedicated for that purpose. We'd recommend that your first course should be your CCNA. It's not advisable to launch directly into a CCNP as it is very advanced - and you need to work up to it before you take this on.
  • Microsoft SQL Computer Training In The UK Explained - By: Jason Kendall
    Everybody is busy these days, and most often should we have cause to advance our future prospects, getting educated outside of working hours is our best way forward. Certified training from Microsoft can fill that gap. You might like to find a training advisor, who could offer counsel on which job role would suit you, and what sort of duties are suitable for somebody with your abilities and personal preferences. Be sure your course is matched to your ability level and skills. A quality company will always guarantee that the training is purpose built for the career you want to get into.
  • Thinking About Web Design Courses Uncovered - By: Jason Kendall
    If you're considering a web design career, then you need training in Adobe Dreamweaver. We also advise that students get an in-depth understanding of the entire Adobe Web Creative Suite, which incorporates Flash and Action Script, to be able to facilitate Dreamweaver professionally as a web designer. Having such skills can lead to becoming an ACP (Adobe Certified Professional) or an ACE (Adobe Certified Expert).
  • Home Based CompTIA A Plus Training - Insights - By: Jason Kendall
    CompTIA A + has a total of four exams and study sections, but you only need to get your exams in 2 of them to qualify for your A+. For this reason, a great number of colleges simply offer two. Yet learning about all 4 will help you to build a far greater perspective of it all, something you'll discover is a Godsend in the commercial world.
  • MCSE Career Retraining - News - By: Jason Kendall
    Are you toying with the idea of doing an MCSE? Then it's likely that you'll fall into one of the following categories: You're currently an IT professional and you should formalise your skills with an MCSE. In contrast this could be your first step into the computer workplace, and your research tells you that there are many opportunities for qualified people.
  • Home Study Adobe Web Design Training Examined - By: Jason Kendall
    Should you be considering getting into the web design industry, studying Adobe Dreamweaver is a fundamental criteria to achieve professional credentials that are globally recognised. To utilise Dreamweaver commercially as a web designer, an in-depth and thorough understanding of the complete Adobe Web Creative Suite (which includes Flash and Action Script) is in our opinion essential.