Redistributing Into OSPF

[Rubicone]

I need some clarification on the following statement:

"Since OSPF is a classless routing protocol, it will accept routes that are not of classful boundaries. In order for OSPF to accomplish this, you need to append the keyword subnets to the end of the redistribution line. The subnets keyword will allow for classless routes to be redistributed into OSPF. Without the subnets keyword, OSPF will only accept classful routes and will reject classless routes."

Where is the confusion you may ask? According to the first sentence OSPF will accept routes from classless boundaries. Yet in the third sentence keyword subnets is required for classless routes to be distributed into OSPF. With the last sentence reading that only classful routes will be accepted. If OSPF is a classless routing protocol why would it not accept classless routes to begin with?

 

[wyvrn]

Well the first sentence probably could be worded better. It looks like OSPF by default is not in classless mode, therefore you have to run the keyword subnet to activate classless routing. In other words it is capable of classless routing, but by default is not run in classless mode.

Are you studying for a cert exam?

 

[Rubicone]

Found something on Cisco's site for those interested. http://www.cisco.com/warp/public/104/3.html#14.0

(BTW the answer to your question is yes. Have you written the new BCSI exam by chance?)

 

[spidey07]

OSPF has no real understanding of class. It is a classless routing protocol.

So when you take routes from another process/protocol (even if that process IS a classfull protocol) you have to tell OSPF what to do with them. By default it will take the routes and assign a classfull mask. there are other variables associated with routes that you can set as well like route type and metric and filters to help with redistribution.

Hope this helps. If you want to learn more then read the RFC. Most anything you want to know about a protocol is contained in them.

 

[wyvrn]

I just passed the CCNA, debating on whether to go for CCNP. Since nobody is knocking down my door with an offer (I have classroom experience only), I have put it off. I love routers and switches though, I would almost take the CCNP for fun but I need to finish my bachelor's first.

Originally posted by: Rubicone
Found something on Cisco's site for those interested. http://www.cisco.com/warp/public/104/3.html#14.0

(BTW the answer to your question is yes. Have you written the new BCSI exam by chance?)

Nothing like an answer from an experienced tech!

OSPF has no real understanding of class. It is a classless routing protocol.

So when you take routes from another process/protocol (even if that process IS a classfull protocol) you have to tell OSPF what to do with them. By default it will take the routes and assign a classfull mask. there are other variables associated with routes that you can set as well like route type and metric and filters to help with redistribution.

Hope this helps. If you want to learn more then read the RFC. Most anything you want to know about a protocol is contained in them.

 

[spidey07]

OSPF is actually my specialty. I actually like working with it.

But anyway more tidbits on OSPF. It is a classless routing protocol because for each network, route, link, router or external in its database there is also a network mask.

So with the subnet mask information there is no need to go by class. The reason why some routing protcols like RIP and IGRP are classfull is because in the routing updates, there is no mask field in the packets. Only networks are sent out, no mask.