OSPF network type

[arun.mallappally123]

Hi All ,

Please tell me any field is there in OSPF packets  to specifty the network type .How OSPF is detecting network type mismatch.

For ex: one side having point-to-point and other side is broadcast.

thanks,

Arun

[Rolf Fischer]

Hello Arun,

regarding mismatch detection, the OSPF network types difer in types that elect DR/BDR vs. no election, "fast" and "slow" timers and in addressing multicast vs. unicast (static peering).

Another difference is which next-hop IP is used, original vs. advertising router.

Because of that its e.g. possible to combine point-to-point and point-to-multipoint when you adjust the hello/dead timers (which makes little sense outside the R&S Lab exam where they expect you to do things like that).

http://blog.ine.com/2008/01/08/understanding-ospf-network-types/

Hope this helps

Rolf

[Jose Jara]

Fischer,

you are right about the mismatch detection. However, I disagree about the possibilities of combining different OSPF network types, even though the most common approach is to use the same. For example, in DMVPN Hub and Spoke Phase 1, you could use point-to-multipoint in the Hub and point-to-point in the Spokes. You could use point-to-multipoint in the Spokes as well, however, I think it is better in this case the point-to-point in the Spokes as they would not advertise the host routes created by point-to-multipoint network type. Of course, for this to work you should adjust the timers.

Best Regards,

Jose.

[Rolf Fischer]

Hi Jose,

thanks for giving me a real-world example.

To tell the truth, I don't see the advantage of the LSDB representation of P2P (Link and Stub network) over P2MP network yet but I'll think about it.

Best regards

Rolf

[Jose Jara]

The advantage is that the spokes will not have the host routes injected by other spokes due to the point-to-point network type, which would be useless in this design (data traffic must go through the hub). Therefore, reducing the RIB size.

[arun.mallappally123]

Hi ,

Thanks for your reply,

I found the following points when I was doing the LAB .For ex: One side Broadcast and another side point-to-point.In this case broadcast side will show as FULL/DR and point-to-point side will show as FULL/- . LSA will propagate b/w the neighbors but the routes  will not get injected in to routing table .Is that means OSPF can't able to detect the N/W type mismatch b/w neighbors.

in any way OSPF can detect N/W type mismatch .

[Rolf Fischer · Accepted solution]

Well, it depends. At least timers and addresing types have to match.

In this case broadcast side will show as FULL/DR and point-to-point side will show as FULL/-

That's true. Exchanging hellos will work bidirectional but then the P2P-router doesn't send it's updates to AllDRouters (224.0.0.6), which the DR is waiting for.

Best regards

Rolf

[arun.mallappally123]

Hi  Fischer,

thanks for the reply .

So this is my conclusion.

OSPF can't detect the N/W type mismatch and can't give any error messages because non of the OSPF packet is  having N/W type field to specifty it.But SPF algoritham will fail and routes will not get injected in to the routing table

Please correct me if iam wrong

thanks in advance.

[Rolf Fischer]

Arun,

thank you.

But I have to make one correction of my last posting:

 but then the P2P-router doesn't send it's updates to AllDRouters (224.0.0.6), which the DR is waiting for.

Much more important is the fact, that from a LSDB perspective, the advertising router is not reachable, thus the prefixes won't be installed in the routing table. The database prepresentation of P2P (P2P-Link and a Stub network) differs very much from a Multicaccess network, you can see that with "show ip ospf database router".

Reference: http://www.ietf.org/rfc/rfc2328.txt Figure 1a and 1b

Thanks again,

Rolf