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

This excerpt is taken from the link below..

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

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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

Thanks ! Sundar.

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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

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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.

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