ospf and area 0

Unanswered Question
May 10th, 2008


I have three routers. R1 is in area 1 , R2 is in area 1,2 and router 3 is in area 2. I can see routers from area 1 in area 2, so why do i need area 0? please can someone tell me the significance of area 0?

I have this problem too.
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waleed_amer Sat, 05/10/2008 - 05:15

Hi friend,

SPF can prevent loops inside the area but what about loops between areas so we need to connect any area to area 0 only so any area route has to pass through area 0 before go to another area so we can prevents loops between areas (except cases that we can use virtual-link).

Regards, W.Amer

NAVIN PARWAL Sat, 05/10/2008 - 05:21

Amer thanks does it mean that I can have multiple ospf areas without area 0 and it will work? ofcourse it would be a bad design, cause I was able to put up come areas and get routes amonst them without area 0.


mattcalderon Sat, 05/10/2008 - 05:26

SPF area 0 is really a conceptual and design issue. Yes you can have ospf work without the use of area 0, but as the said above you need area 0 from a design perspective. You need to have that backbone area that all other areas traverse through. All areas should connect back to area 0. This is where the idea of ABR/ASBR routers begin to take place.

Harold Ritter Sat, 05/10/2008 - 06:49


This doesn't sound right. Can you post the output from "show ip ospf" from R2.


Richard Burts Sat, 05/10/2008 - 08:17

If I understand the topology correctly R2 is in both area 1 and area 2 and should have the full set of routes for both areas. The real question is whether R1 has any routes from area 2 and whether R3 has any routes from area 1.

I have tested this kind of thing before and found that the router in a single area (R1 and R3) do not learn routes from the remote area unless the router in both areas has some area 0 (perhaps on a loopback) or if the router in both areas is doing something creative like having 2 OSPF processes and redistributing between them.

I agree that the output of show ip ospf from R2 would do a lot to help us see what is going on.



Harold Ritter Sat, 05/10/2008 - 10:20


Here's some more information. As Rick stated, there has to be at leats one interface active in area 0 for the router to see itself as an ABR, as menrioned in RFC3059 section 2.1:

Area Border Router (ABR):

Cisco Systems Interpretation (of an ABR):

A router is considered to be an ABR if it has more than one area Actively Attached and one of them is the backbone area.

And in the same section:

Actively Attached area:

An area is considered actively attached if the router has at least one interface in that area in the state other than Down.

Cisco IOS implementation conforms to this RFC and would therefore consider any given router, such as R2 in your case, as an ABR if and only if there is is at least one interface in area 0 (could be the loopback interface) and this interface is in a state other than down.

The output of "show ip ospf" would tell us if the router sees itself as an ABR.

For more information, please refre to RFC3059:



bvsnarayana03 Sun, 05/11/2008 - 11:59

Harold, correct me if I'm wrong. An ABR is a router which has atleast 2 areas connected to it & are non-zero. When a router connects to area 0, its called a Backbone router.

In this scenario where a router is connected to both area 1 & 2 & maintain separate LSDB for both areas,will the 2 areas communicate with each other without the presence of area0.

Lets suppose this router doesnt run any routing protocol, then with one interface in each subnet it could stil route between them via ARP. But if we run OSPF now with each interface in separate area, wont they still communicate or now they require area 0?

Richard Burts Sun, 05/11/2008 - 13:55


I thought that Harold was quite clear in his explanation when he said:

Cisco Systems Interpretation (of an ABR): A router is considered to be an ABR if it has more than one area Actively Attached and one of them is the backbone area.

In Cisco's implementation if a router has interfaces in 2 areas but not in the backbone then the router is not considered an ABR.

In the original post the question asked about a scenario in which R2 was in area 1 and in area 2. Therefore R2 will be able to route to destinations in both areas. But in Cisco implementation R2 should not advertise area 1 LSAs to the router in area 2 and should not advertise area 2 LSAs to the router in area 1.

Whether a router runs a routing protocol or not it should always be able to route between its connected subnets. It does not matter what areas they are in, the router can route between connected subnets.




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