Cisco OSPF Question

[jorma]

I have a single area and am running OSPF.

Are all networks expected to have their own entry in the link-state database? If not, what is the criteria for having one, and also what is the impact if a given network does not have its own entry.

In my case I've got some connections between routers using ethernet, using /30 subnet masks. If I config the interfaces as "ip ospf network point-to-point" then the thing shows up as a stub network in the LSAs of the routers connected to it, but it does not get its own entry in the database. If I configure it as "ip ospf network broadcast" then it gets its own entry.

Also, when configured as p-to-p it is listed as a stub network - what is the significance of this, being that I have only a single OSPF area?

My biggest concern in all of this is that I'm afraid that networks which do not have their own LSAs may not be used in calculating routes. I guess I'm thinking that every network in the area should have an entry in the LS DB.

BTW the network design was handed to me, I did not come up myself with the idea of using ethernet for p-p links.

Thanks a lot

 

[wybnormal]

A stub area is a good idea if you can use it.. the routers inside will exchange LSAs within themselves.. they will get INTRA-area routes but not EXTERNAL routes.. the ABR will have a default route to use like 0.0.0.0 0.0.0.0

The main benefit is the stub routers will not have to have all the routes so they will use less memory. With a single default route out, it makes things a bit easier to configure.

There are good details in Cisco Academy semester5, CCNP Routing Exam Guide and Cisco's website.

MikeS
Find me at

http://www.packetattack.com

"The trouble with giving up civil rights is that you never get them back"

 

[nilesh74in]

Hi

You said u have single area means area 0..right.
Network entries are distributed by DR on particular network segment to all routers..and all network entries must be there unless u don't have summarised route.


When u configure a P-to-P network that means its not a multiaccess network and it is a stub. So no need to configure it as a stub.
Also u have only one area.

Nilesh
visit us at:

http://netconsult.8k.com

 

[wybnormal]

Hey Nilesh-- becareful with the assumption of area 0.. I was at a place once where the techs did not understand OSPF when they config it.. there was not ANY area 0, just area 1 which functioned pretty much the same. But I could never get a straight answer out of Cisco if it really made a big difference. Now I think it would in some of the more esotric configs.. like virtual links, stubbing off several areas etc. I know some management software made the assumption that area 0 would be there.

Never the less, it was unsettling to see the config...

MikeS
Find me at

http://www.packetattack.com

"The trouble with giving up civil rights is that you never get them back"

 

[jorma]

Thanks - this to me raises a few additional questions.

1. An ethernet LAN is certainly multiaccess, but what about when there is nothing on this LAN but 2 routers,
with a 30 bit mask. While the medium is multiaccess, the topology is point to point. Does the medium or the topology decide whether to use "ip ospf network point-to-point" or "ip ospf net broadcast"?

I think that typically you would not have such a setup, so this problem would not normally come up, and we would not have to question very hard whether to use point-to-point or broadcast, etc. Again, I did not come up with this topology, it was handed to me and not expected for me to change it.



2. This topology is basically a mesh of routers connected by ethernet segments. It is not FULLY meshed, i.e. every router is not directly conencted to every other router. In fact almost all the ethernet segments have nothing but the 2 routers on each end, and nothing else on the segment. So it's essentially a bunch of "point to point" ethernet links. It is NOT a heirarchical topology.

Given this, how does the concept of stub networks fit in? Every single link is expected to be used as a route when it provides the shortest metric route to the destination. And another thing is that links which do not have their own entries in the LS DB actually DO get used to route traffic when they are the best choice.

Thanks again