Theoretical OSPF question

[John Blakley]

All,

I'm playing with ospf, and I noticed something. I have two routers that are connected into a switch.

RouterA is 192.168.1.1

RouterB is 192.168.1.2

RouterA's ospf configuration:

router ospf 1

network 192.168.1.0 0.0.0.255 area 25

network 1.1.1.1 0.0.0.0 area 0

area 25 nssa no-summary

RouterB's ospf configuration:

router ospf 1

network 192.168.1.0 0.0.0.255 area 25

network 100.100.100.100 0.0.0.0 area 25

area 25 nssa

On RouterA, I do a "sh ip ospf neighbor detail" and I see that the BDR is 192.168.1.2 (NSSA) and the DR is RouterA.

Since type 7 LSAs stay in the NSSA, if the DR (RouterA) failed, what would happen to RouterB? It wouldn't take the configuration and automatically put itself into area 0. So, I'm assuming that this is why we use priority to force a router to be the DR, but what happens if someone forgot? Would all routing be screwed up?

Thanks,

John

[John Blakley]

Okay :) I think we're crossing posts now because I just asked you questions that you've answered here.

Thanks,

John

[Edison Ortiz]

Ok, let me expand further with a diagram (see attached).

Router A and Router B are the ABR routers.

Router A is the DR on its connection to Router B while Router B is the BDR for that segment.

Now, Router A also has a connection to Router C but on this connection Router A is the BDR for that segment.

If you noticed on the right side, there is also area 1 but there is a separate DR/BDR election because is another broadcast segment, but they are on the same area.

Hope this help you understand a bit better.

[John Blakley]

That helps with the concept of DR/BDR for me.

Questions:

Does the DR send LSAs when there is a change in the network?

If yes, LSAs in an NSSA are sent as type-7s, right?

If that's correct, if the DR in an NSSA (again, not sure if it cares) sends LSAs out because of a change in the network, does it:

send to all areas, or at least into its own area, and to area 0 to propagate out to other areas?

does it get translated when it hits area 0 as a type-5 by the ABR

does it even care?

Thanks,

John

[Edison Ortiz]

Does the DR send LSAs when there is a change in the network?

All OSPF routers do, it's a link state protocol.

If yes, LSAs in an NSSA are sent as type-7s, right?

Only external LSAs.

If that's correct, if the DR in an NSSA (again, not sure if it cares) sends LSAs out because of a change in the network, does it:

send to all areas, or at least into its own area, and to area 0 to propagate out to other areas?

A DR is only in charge of sending updates to other routers participating in the same broadcast domain area. If there is another router in that area and this router is not a DR but it has multiple areas in it, this router may be the one propagating the information. Again, apples and oranges..

[Giuseppe Larosa]

Hello John,

as Edison has noted the DR election is local to a network segment in an OSPF area.

Notice also that having a DR router that is not an ABR is allowed by OSPF and in some cases it can be recommended to have some load sharing:

the ABR has already the duties of inter-area routing, double LSDB and so on.

Having the ABR to be also the OSPF DR in one or multiple segments can make the ABR heavy loaded with OSPF tasks with other routers sitting idle.

so the position of the router in the network is not considered.

From the point of view of another area it is the ABR that builds the LSA type 3 for the ip subnet of the LAN segment.

the DR is responsible of creating and mantaining the network LSA type 2 that it is only flooded in the original area.

For this reason the ABR don't need to be also the DR: ospf data structures allow for this.

Hope to help

Giuseppe