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Theoretical OSPF question

John Blakley
VIP Alumni
VIP Alumni

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

HTH, John *** Please rate all useful posts ***
19 Replies 19

rais
Level 7
Level 7

For OSPF, you don't have to have area 0. Area 0 is required only if you have more than one area. So B would simply work in area 25.

Thanks.

OSPF requires area 0 to be it's backbone, and you either have to have a router with an interface in area 0, or you need to be connected to a router that has an interface in area 0.

*Edit* Rais, thanks for the post. I didn't realize that single areas didn't need area 0. I learned something today :)

John

HTH, John *** Please rate all useful posts ***

John

As Rais stated there is nothing in OSPF that dictates you must use area 0. If you have multiple areas then you need an area 0 as the backbone area but as B only has interfaces on area 25 if A failed B would simply run in area 25.

Jon

Well,

I just tested this, and against everything I've read, it works without area 0. EVERYTHING I've read states that you need area 0 for OSPF to work correctly.

John

HTH, John *** Please rate all useful posts ***

But you can run OSPF in a network with a single non-0 backbone area.

You need an area 0 backbone area when you plan to have multiple areas. Going from one area to another needs the existence of the area 0 whereas on single area design, you can use any number.

Best practice is to always have an area 0 but it's not mandatory on single area design.

__

Edison.

Good post Edison. I tested, and found this to be true (non-0 single area). I get routes like I should be.

Thanks,

John

HTH, John *** Please rate all useful posts ***

Edison Ortiz
Hall of Fame
Hall of Fame

A DR/BDR assignment does not make the router become part of area 0 (backbone area).

The area placement is done on the network command under the OSPF process.

Per your example, RouterA is the only router holding area 0 and RouterB won't dynamically become part of area 0 without you adding a link into this area with the network command under OSPF.

HTH,

__

Edison.

Edison,

I'm wondering (maybe because it's on a broadcast domain) why the NSSA was chosen as the BDR. I can even force it as the DR.

Thinking along those lines, since all DROthers associate to the DR, why would OSPF allow for an NSSA to be the DR in the first place?

John

HTH, John *** Please rate all useful posts ***

The DR/BDR assignment is chosen based on the router-id (highest win), or the router with the highest ospf priority, also you must understand the first router that boots becomes the DR and OSPF does not have any DR preemption, so if you have a fast router in a broadcast domain segment, that router must likely will be the DR.

You can avoid this by having ospf priority 0 in all routers that you don't want to participate in the DR election.

Right. I guess I'm not getting this question across correctly.

What happens if the DR dies in one area, and the BDR happens to be in another area? The BDR becomes the new DR, right? Now how would the "new" DR work if the new DR was in a stub area.

Again, this is all theoretical.

Thanks,

John

HTH, John *** Please rate all useful posts ***

John,

There are 2 things:

DR/BDR will come into play when you are in Broadcast or NBMA network. It has its own method to elect who is going to be DR or BDR.

NSSA,This means that you don't want to see any LSA type5 in this area. Including summary keyword. It means you don't want to see LSA type3 and type4 as well.

Yes in case of Broadcast/NBMA. When you lost DR then the BDR should take this role instead. If there are other routers there, Well someone will be elected to be (new)BDR.

Toshi

If the DR dies in one area (broadcast segment) and you have other OSPF speaking routers in the same area (broadcast segment), you will have a BDR on that area.

You won't have a DR in one area and a BDR in another area.

A router can be a DR in one area and a BDR in another area at the same time.

You are mixing OSPF Network Types with OSPF Area Types.

"A router can be a DR in one area and a BDR in another area at the same time."

I'm assuming that's what I was seeing. The DR was in area 25, and the BDR was in areas 0 and 25.

"You are mixing OSPF Network Types with OSPF Area Types."

I didn't think I was. I wanted to know how a stub network would act if it was the DR for a segment like the scenario I had. (Maybe that's mixing it, and the DR/BDR scenario doesn't care about how the areas are configured.)

I think my main question was how the lsas that come from a DR that was in a nssa would be affected.

John

HTH, John *** Please rate all useful posts ***

It uses the NSSA standards rules as any external route (LSA Type 5) will be translated (LSA Type 7).

A DR will not necessarily be the ASBR for the NSSA area.

Remember, NSSA Area can be spread around multiple broadcast domains. Each broadcast domain will have a DR/BDR.

That's the reason I keep telling you, you are mixing two concepts.

__

Edison.

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