Cisco CCNA / CCNP Certification: OSPF ASBRs Explained And Illustrated

When I first started studying for my CCNP, some of the concepts of OSPF really confused me. This was especially true for the ASBR, stub areas, and total stub areas. Sure, I could memorize the LSA types associated with these devices and area types, but there just weren't any illustrations that explained exactly what was going on.

This CCNP tutorial shows an ASBR in operation, and also introduces you to a basic example of route redistribution. Don't worry, it gets more complicated. :)

Here's the network we'll be working with in this tutorial:

R5

|

R1

/

R2 R3

Networks:

R1 - R5 Ethernet Segment: 10.1.1.0 /24

R1 - R2 - R3 Serial Segment: 172.16.123.0 /24 (Preconfigured with OSPF)

R1 and R5 are running RIP over their common Ethernet segment, 10.1.1.0 /24. R5 has three loopbacks it will be advertising into the RIP domain.

R1 is also running OSPF, with R2 and R3 as neighbors. Even though R1 knows about the loopbacks on R5, its OSPF neighbors do not. R1 has these routes in its RIP routing table, and for OSPF neighbors to learn of these routes, route redistribution must be manually configured.

R5#conf t
R5(config)#router rip
R5(config-router)#version 2
R5(config-router)#no auto-summary
R5(config-router)#network 5.0.0.0
R5(config-router)#network 10.0.0.0

R1#conf t
R1(config)#router rip
R1(config-router)#version 2
R1(config-router)#no auto-summary
R1(config-router)#network 10.0.0.0

R1#show ip route rip
5.0.0.0/24 is subnetted, 3 subnets
R 5.1.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0
R 5.2.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0
R 5.3.1.0 [120/1] via 10.1.1.5, 00:00:10, Ethernet0

R1 has a route for all three of R5's loopback interfaces, learned via RIP. R1 is also running OSPF, but its neighbors R2 and R3 don't know about these RIP routes:

R2#show ip route ospf

R2# < no output from show command means no routes! >

R3#show ip route ospf

R3#

Be careful when configuring redistribution use IOS Help to make sure you're not missing any vital options. IOS Help shows that there is a "subnets" option when redistributing RIP into OSPF. If that is left off, only classful networks are redistributed (as the router is kind enough to tell us). In this case, we have no classful networks, so there will be no redistribution. R2 will not see the RIP routes.

R1(config)#router ospf 1
R1(config-router)#redistribute rip ?

metric Metric for redistributed routes
metric-type OSPF/IS-IS exterior metric type for redistributed routes
route-map Route map reference
subnets Consider subnets for redistribution into OSPF
tag Set tag for routes redistributed into OSPF

R1(config-router)#redistribute rip
% Only classful networks will be redistributed

R2#show ip route ospf

R2#clear ip route *

R2#show ip route ospf

R2#

On R1, we'll now use the "subnets" option, and the RIP subnets are successfully redistributed into OSPF.

R1(config)#router ospf 1

R1(config-router)#redistribute rip subnets

R2 now sees the redistributed routes.

R2#show ip route ospf

5.0.0.0/24 is subnetted, 3 subnets

O E2 5.1.1.0 [110/20] via 172.12.123.1, 00:00:08, Serial0.123

O E2 5.2.1.0 [110/20] via 172.12.123.1, 00:00:08, Serial0.123

O E2 5.3.1.0 [110/20] via 172.12.123.1, 00:00:08, Serial0.123

10.0.0.0/24 is subnetted, 1 subnets

O E2 10.1.1.0 [110/20] via 172.12.123.1, 00:00:08, Serial0.123

Notice that the routes are marked as "E2". The E indicates that these are external routes these are routes that were learned by OSPF via redistribution.

Naturally, you'll want to ping these networks to make sure you have two-way communication. Even though the routes to these loopbacks are in the routing table, pings fail:

R2#ping 5.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 5.1.1.1, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

Keep this in mind when troubleshooting: It's not enough to have a route to a destination for pings to succeed there's got to be a path back. At this point, R5 doesn't know where the 172.12.123.0 network is, so there's no way the ping can get back. More route redistribution is necessary on R1.

Your first instinct may be to redistribute OSPF routes into RIP to make this work, but remember that the 172.12.123.0 /24 network isn't known to R1 via OSPF it's a directly connected network. We can redistribute connected networks into RIP to give R5 a route to 172.12.123.0 /24, which will give the ICMP packets a path back to R2, which will allow pings to succeed.

R1(config)#router rip

R1(config-router)#redistribute connected

R5#show ip route rip

1.0.0.0/32 is subnetted, 1 subnets

R 1.1.1.1 [120/1] via 10.1.1.1, 00:00:02, Ethernet0

172.12.0.0/24 is subnetted, 1 subnets

R 172.12.123.0 [120/1] via 10.1.1.1, 00:00:02, Ethernet0

R2#ping 5.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 5.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 68/69/76 ms

Performing this redistribution makes R1 an ASBR, as shown here with show ip ospf. This command even shows you what protocol(s) are being redistributed into OSPF.

R1#show ip ospf

Routing Process "ospf 1" with ID 1.1.1.1

Supports only single TOS(TOS0) routes

Supports opaque LSA

It is an autonomous system boundary router

Redistributing External Routes from,

rip, includes subnets in redistribution

Knowing exactly what the ASBR does and how to configure route redistribution are just a few of the many OSPF skills you'll need to earn your CCNP. Like any other Cisco skill, breaking a complex topic like OSPF down into smaller, manageable pieces is the best way to master these topics and pass the CCNP exams.

Keep watching The Bryant Advantage website for more free CCNA and CCNP tutorials!

To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage. The Bryant Advantage offers dozens of FREE CCNA, CCNP, and Home Lab tutorials and articles. The Bryant Advantage sells the world's most comprehensive CCNA Study Guide, and my exclusive Binary Math and Subnetting Mastery book. My CCNA Online Fast Track is the world's fastest-growing CCNA course. Get your CCNA and CCNP today at http://www.thebryantadvantage.com

---

Is Your Web Browser Putting You At Risk?

It's free, it comes with Windows and it's used by... Read More

Freezing - Time To Warm Up Your PC

Freezing is also known as crashing or hanging. It's frustrating.... Read More

Desktop Security Software Risks - Part 2

This is the third in a series of articles highlighting... Read More

Dynamite Comes in Small Packages - Tiny Personal Audio MP3 Players Pack Powerful Music Enjoyment

MP3 players are Hot! Playing music has come a long... Read More

Buying a Personal Computer (PC)

For many people the computer industry is a seeming unsolvable... Read More

The Newbies Guide to Personal Computer Maintenance

When you turn on your computer, does it act like... Read More

Computer Geeks and Garden Gnomes

First and foremost before I begin my ranting it is... Read More

Buying a Home Theater Receiver

Buying a receiver is one of the most important decisions... Read More

Connect Your IPAQ to Linux

Choose Not To ConformPalm pilots and pdas are constantly growing... Read More

Selecting the Perfect Big Screen TV

Selecting a TV isn't as easy as it used to... Read More

USB Drives - What to Look For in a USB Device

Those small USB drives have so many names, pocket drives,... Read More

What Does That Error Message Really Mean?

Surf the 'Net for about 10 minutes and chances rate... Read More

How to Buy a Plasma Television Set

Most people think that all you have to do to... Read More

The Benefits of the New Firefox Browser

You probably heard of the new Firefox browser version 1.0... Read More

Is Online DVD Rental or Pay-Per-View the Best Way to Get Your Movies?

With renting methods such as online DVD rental and pay-per-view,... Read More

How To Speed Up Your Aging PC And Make It Healthier Too

With a little tweaking and with proper tools, you can... Read More

10 Time Saving Keyboard Shortcuts

Time is money. And when you constantly have to divide... Read More

How Does My PC Get Hot

There are many sources of heat that can raise the... Read More

A Beginners Guide to Avoiding Viruses

"Aaaaaahhhhhh! I've been invaded by a virus!" Getting a virus... Read More

5 Simple Ways To Increase Your Computer Speed & Help Your Computer Run More Efficiently!

If you're a computer expert, you'll probably already know about... Read More

Cisco Certification: Building Your Own Home Lab, Part I

CCNAs and CCNA candidates hear it all the time: â??Get... Read More

Cisco Certification: The Most Important Cisco Study Youll Ever Do

All of us are familiar with the pyramids of Egypt.... Read More

Printing Multiple Copies of Photos

I do a holiday letter every year and send them... Read More

Tips For Getting Technical Support Help Online

Sometimes your PC will start acting strange for no apparent... Read More

MCSE 70-290 Certification Primer

Microsoft Certifications are one of the most widely acclaimed, pursued,... Read More

How To Become A True CCNA

I've worked my way from the CCNA to the CCIE,... Read More

FTP - File Transfer Protocol Explained

File Transfer Protocol (FTP) is a protocol that is part... Read More

Cut Through the Hype and Make the Right Digital Camera Comparisons

There are soooo many choices. With all the styles and... Read More

Cache In Your Chips And Get A Bus!

Computer related terminology could sometimes be daunting to newcomers. These... Read More

Your Home is in Your Hands

I would like to tell you about a magic home.... Read More

A Peek Into the Near Future of Electronics Technology

How long do you think DVDs have around? 20 years?... Read More

The Benefits of Open Source

If you are a building a website or a forum,... Read More

More Cool Web Tricks

If you ask most people what frustrates them most about... Read More