Default Route Propagation

[turbo_engine26]

Hello,

So, we have 3 different ways to configure and propagate a default route:

1- Using static default route (ip route 0.0.0.0 0.0.0.0 next_hop) then redistributing it for propagation (redistribute static)

2-  Using the ip default-network x.x.x.x. This command by itself will  propagate the network specified as an argument in the command with some  exceptions. These exceptions are;

a. RIP will receive the default route as 0.0.0.0 (not as the network specified)

b. IGRP or EIGRP will receive the default route as the network specified.

3- Using static default route then propagate it using the default-information originate [always] with OSPF or IS-IS

Now,  i think there is a mistake in the "CCIE Practical Studies vol. 1" book,  please see attached snapshot. (the configuration in the rectangle)

It used both the default-information originate and ip default-network. OSPF and IS-IS do not support the second command.

Do you beleive it is a mistake in the book? .... It confuses me big time.

Regards,

AM

[Richard Burts]

I agree with you that the default-network should not be included if the router is using just OSPF (as seems to be the case, but since we do not see the entire discussion I can not be sure).

HTH

Rick

[turbo_engine26]

Hello,

Thx for your reply.

What did you exactly mean by " not seeing the entire discussion"? ... Did you mean the full scenario?

Regards,

AM

[Richard Burts]

AM

Yes. What I meant is that I only see part of one page. Without knowing what else was in that section I can not know for sure that the intent was that the router runs only OSPF. It seems likely that the router runs only OSPF, but I can not know for sure.

HTH

Rick

[turbo_engine26]

Sure, Understood.

Please see attached topology screenshot. Let me know if you can see the image clearly.

I want to accomplish the last bullet.

Regards, AM

[Richard Burts]

AM

I can see the image fairly clearly. You get OSPF to propagate the default route using default-information originate. So the remaining question is how would you want to have OSPF recognize the default route? What do you think the answer(s) might be? (is there more than one way to have OSPF recognize the default route?)

HTH

Rick

[turbo_engine26]

I appreciate not to answer a question by a question and let's solve the silly maze.

All i need to know if the configuration in that book, right or wrong. Not a big deal, huh? ..  I think i mentioned my point already by saying that the default-information originate is used by OSPF and IS-IS to propagate the default route. The book used both commands: ip default-network and default-information originate. So i asked why the book included both of them.

Forigve my ignorance, i am not a master of geeks like you as i am still learning this stuff.

Regards,

AM

[ebarticel]

Hi AM,

The simple explanation that I have is that "ip default-network" is an IOS command (same as ip default-gateway) and "default-information originate" is a routing protocol specific command.

Hope this helps

Eugen

[Richard Burts]

AM

Since you asked a question from the book CCIE Practical Studies I assumed that you were studying for the CCIE. Perhaps that is not the case.

One thing that is important in studying for the CCIE is to recognize that there are frequently more than one way to achieve an objective. And in this case there is more than one way to get OSPF to advertise the default route. I believe that the example given in the book would work ok. It uses the default network command to get the local router to recognize the default network. And once the local router knows the default route then the OSPF default-information originate would work. I believe that you could also get it to work by not using default network and configuring a static default route which would also cause the OSPF default-information originate to advertise the default route.

HTH

Rick

[turbo_engine26]

One thing that is important in studying for the CCIE is to recognize that there are frequently more than one way to achieve an objective. 

I absolutely know this fact, that's why i posted this topic in the first place. If you noticed my first thread, i mentioned 3 different ways to advertise the default route. This indicated that i am fully aware about this fact.

In "Routing TCP/IP Vol. 1", the author didn't mention if the default-network command can be used with OSPF. Instead, he just simply stated that any routing protocol doesn't support this command won't work. Also, this book mentioned to use the static default route (0.0.0.0) for the local router to recognize it then use default-information originate to advertise it within the OSPF domain. Therefore, when i saw the configuration in "CCIE Practical Studies" book, i got confused because practically it didn't work in GNS3 and the router didn't recognize the default route using the default-network command.

Actually, this is one of the challenges that a person could face when studying from multiple resources. Every author use his/her own unique way to accomplish an objective but the concepts are the same.

Regards,

AM

[turbo_engine26]

Sorry, i need to correct something.

I mean the local router itself recognized it but when that router advertised it, the other routers in the domain didn't recieve it.