OSPF injected default route

Unanswered Question
Feb 27th, 2007

Hi Expert,

What is the different from inject a default route from ASBR ( default-information originate) into ospf domain compared with manually configure ( ip route 0.0.0.0 0.0.0.0 next hop ) to all ospf router ? Does the purpose is the same ? Just default-information originate is generate the default route and propagate to ospf domain automatically and ip route 0.0.0.0. 0.0.0.0 is manually.

I have this problem too.
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rais Tue, 02/27/2007 - 17:48

With default-information originate your OSPF ASBR will propagate default-route only if it has one in its routing table. Note that always option makes default route advertisement unconditional.

Manual configuration will be fixed and will not change with changing network topology.

Hope this helps.

sundar.palaniappan Tue, 02/27/2007 - 17:55

Though the goal is the same the former choice, injecting a default route via OSPF, is an efficient way of doing it.

I shall try to explain this with a small example.

R1-->R2-->R3-->Internet

In this scenario, R3, by virtue of it's connection to the Internet, is advertising the default route via OSPF or another IGP to the rest of the domain and everyone routes through R3 to get to the Internet. Let's say, the connection to the Internet from R3 is down then R3 would stop advertising the default route. A packet destined to the Internet arriving on R1 will be dropped right there as R1 wouldn't have a default route that it could use to forward the traffic.

On the other hand, if you were using static routes on all 3 routers then when the Internet link from R3 is down both R1 & R2 would keep sending packets to R3 and R3 would eventually drop it. As you can see, this setup causes packets to traverse links and hops unnecessarily only to get dropped at a remote destination.

HTH

Sundar

acbenny Tue, 02/27/2007 - 18:24

Thanks Sundar, I know what you mean, your explain is based on default-information originate (no always). But how about if use "always" which always advertise default route without proper default route configured in ASBR

sundar.palaniappan Tue, 02/27/2007 - 19:42

This excerpt is taken from the link below..

http://cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a74.shtml

#

If the ASBR doesn't have a default route, you can add the keyword always to the default-information originate command (default-information originate always).

This command will advertise a default route into the OSPF domain, regardless of whether it has a route to 0.0.0.0. Another benefit of adding always keyword is that it can add stability to the internetwork. For example, if the ASBR is learning a default route from another routing domain such as RIP and this route is flapping, then without the always keyword, each time the route flaps, the ASBR will send a new Type 5 LSA into the OSPF domain causing some instability inside the OSPF domain. With the always keyword, the ASBR will advertise the default inside the OSPF domain always, and thus the flapping of the default route from the RIP domain will not cause any instability inside the OSPF domain.

HTH

Sundar

acbenny Tue, 02/27/2007 - 20:44

Thanks ! Sundar.

********************************************************

Reference to your posted message (assumed is "default-information originate" is configured in ASBR)

Though the goal is the same the former choice, injecting a default route via OSPF, is an efficient way of doing it.

I shall try to explain this with a small example.

R1-->R2-->R3-->Internet

In this scenario, R3, by virtue of it's connection to the Internet, is advertising the default route via OSPF

or another IGP to the rest of the domain and everyone routes through R3 to get to the Internet.

Let's say, the connection to the Internet from R3 is down then R3 would stop advertising the default route.

A packet destined to the Internet arriving on

************************************************************

Benefit can be concluded as R1 will be dropped right there as R1 wouldn't have a default route that it could use to forward the traffic.

But if according to below example, then this benefit can not be apply, right ?

R1-->R2-->R3-->Internet

If "default-information originate always" is configured in ASBR. Even though ASBR has no default route to outside, R1 still will try to access internet which packet from R1 will travel to R2 and R3 and loop on it.

sundar.palaniappan Wed, 02/28/2007 - 06:32

Hi,

Your are partly correct. The packet would travel to R3 though R3 wouldn't know how to route it and drop the packets but the packets wouldn't loop. As you can see, the packet would end up unncessarily using the bandwidth on those links and R2 & R3's CPU cycle when it could have been easily avoided. You have to be prudent about using the 'always' keyword and one of the situations that you would use it is what was mentioned in the link that I had posted earlier.

HTH

Sundar

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