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

In The News:


pen paper and inkwell


cat break through


Looking For an MP3 Player?

If you don't have an mp3 player, and even if... Read More

System File Checker - A Maintenance Utility

System File Checker is a great utility that is typically... Read More

Flash Memory, Part I: MMC and SD

MMC and SDFlash memory is available in so many formats... Read More

Faster Browsing Tips

IF WE say that it was mainly because of the... Read More

How To Recognize The Telltale Signs Of Spyware

Spyware is the software that collects information about your online... Read More

MCSE 70-290 Certification Primer

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

Apache, MySQL & PHP for Windows

Apache, MysQL and PHP for Windows could be a nice... Read More

Use Your Computer For Your Entertainment Needs

By using your computer today you can find a lot... Read More

Why There Are Color Differences in Printing from Your PC

This article describes the basic properties of color, what the... Read More

How To Buy An MP3 Player

Are you looking for an MP3 player but am not... Read More

Email Management

If you utilize a computer at home or work it... Read More

Cisco Certification: What To Expect On Exam Day

Cisco Certification: Taking Your First Certification ExamYou've studied hard; you've... Read More

Image Formats: GIF, JPEG, BMP

When browsing the internet you are likely to come across... Read More

Learn the Lost Art of Touch Typing

Computers have replaced typewriters, but two-finger, hunt-and-peck typing can never... 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

The Ipod Mini: Why Its Not that Hot

I got my eyes set on a iPod mini, as... Read More

Emulation Manual - A Complete Guide on How to Change Your Windows XP to Mac OS X

IntroductionMac OS X is the most technologically advanced operating system... Read More

Your Computer Cant Keep Time

A computer needs a certain amount of information to operate;... Read More

5 Steps to Remove Spyware for Free

Have you ever tried to remove Spyware off your PC... Read More

Advantages and Guidelines of Automated Testing

"Automated Testing" is automating the manual testing process currently in... Read More

Lives With Games Tradition and Industry

The United States Of America citizen feels that games is... Read More

Spyware Remover

Your first step in removing dangerous infections from your computer... Read More

Put Order and Information into File Names

Unless you place your faith in internal search engine software... Read More

Cisco CCNA / CCNP Certification: OSPF ASBRs Explained And Illustrated

When I first started studying for my CCNP, some of... Read More

An All Too Familar Pain

Lost & Found for the 21st CenturyIn today's hectic world... Read More

Cisco Certification: A Survival Guide To The Cisco Cable Jungle

One of the most confusing parts of beginning your Cisco... Read More

The Newbies Guide to Personal Computer Maintenance

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

HTML Explained: Part 1

Want to save money while promoting your web-based business? Of... Read More

Improve PC Performance - 6 Tips You Must Know

Are you frustrated with your PC?Is it feeling sluggish or... Read More

4 Easy Ways to Speed Up A Sluggish PC

Computers are supposed to speed up our productivity?to help us... Read More

Bluetooth Technology: Tips for Buying Headsets or Headphones

The technological horizon has always got something new to offer,... Read More

Does Microsoft Show Hackers How To Attack?

After another security hole recently surfaced in Microsoft's Windows operating... Read More

Flow Text Around a Graphic in Microsoft Word

Flow Text Around a GraphicQuestion: I have inserted a photo... Read More