OSPF area 0

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Oct 26th, 2007
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We don't have a large network. We used to have three routers at OSPF area 0. But now we plan to replace them by one big, new router. Is it ok if we only have one node in area 0? What interface should be configered as area 0 then, the loopback? Thanks in advance.

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Leandro Nunes Fri, 10/26/2007 - 05:41
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You can use the loopback address as router-id in the process OSPF.

Kevin Dorrell Fri, 10/26/2007 - 05:54
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In OSPF you can use whatever router-id you like. Although it looks like an IP address, it is in fact just any random number. It does not need to be related to the IP address of any interface.


It just so happens that if you do not explicitly specify a router-id, then the router chooses the IP address of a loopback as its "random number". But that is haphazard, and it does so because that is reasonably likely to be unique. That's all.


Kevin Dorrell

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Kevin Dorrell Fri, 10/26/2007 - 05:44
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Yes, that is fine. I presume these three routers were talking OSPF to something else as well, rather than just to each other.


It is interfaces that you add to an OSPF area rather than whole routers. Your area 0 should incude all the interfaces that are equivalent to the interfaces you have in area 0 at the moment. I know that sounds a bit vague, but without any idea of your current architecture, I have no precise idea how to emulate it with your new device.


Kevin Dorrell

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bvsnarayana03 Fri, 10/26/2007 - 05:51
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Generally loopback interfaces are used as router-id. Physical interfaces are added to area 0, the interface which connects to the router in other area.

Cynthia2004 Fri, 10/26/2007 - 05:53
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Thanks all for the reply.


Yes, the three routers talk OSPF area 0 to each other. At the same time, we have three other areas, 1, 2, 3, attached to them. If the three routers in area 0 is going to be replaced by one router, interfaces talking area 1,2,3 will stay in their orignal areas, but interfaces used to talk area 0 will be gone. That's what confused me since I know OSPF must have area 0. So in this case, will loopback interface be the only interface in area 0?

Kevin Dorrell Fri, 10/26/2007 - 06:10
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Yes, that is also OK. The central router will be a border router, with just your loopback in area 0.


Kevin Dorrell

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bvsnarayana03 Fri, 10/26/2007 - 06:26
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Kevin,

Pls help in my understanding.

if the loopback is configured for area 0, how will this router communicate with other routers in same area for forming adjancies.


What about the physical interfaces of this router connecting to area 1,2 &3. In which area they would be connected.


What I understand is, Hello pkt contains area ID. For adjancies to be formed, area IDs on both routers should match. Lets say this router is a part of area 3, putting the loopback to area0 how will it talk to other routers in area3.

Kevin Dorrell Fri, 10/26/2007 - 06:34
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Sorry, perhaps I didn't make myself clear. You are right, the physical interfaces towards the other devices would indeed be in areas 1, 2, and 3.


As I was saying earlier, you add interfaces to an area rather than whole routers. This central router will have one interface in area 0, one in area 1, one in area 2, etc. Since it has interfaces in more than one area, it is an ABR.


Kevin Dorrell

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Richard Burts Fri, 10/26/2007 - 06:43
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B V S


There is a point that has been implicit in this discussion and I think that there might be some value in making it explicit: when OSPF has more than one area then there must be an area 0. It does not matter whether the interfaces in area 0 are physical interfaces or are virtual interfaces. As long as one interface is in area 0 then the router functions as an ABR and it maintains a link state data base for area 0.


So it is ok for a router to configure its loopback interface in area 0 and for that to be the only interface in area 0. The router still has area 0, maintains its area 0 link state data base, and acts as ABR for other areas.


There are at least two ways that the router could be configured in this situation: it can have the physical interfaces configured as belonging to the other areas (area 1, 2, and 3 in this discussion) which makes this router the ABR and the remote routers belong to a single area each. In this configuration for the interface configured in area 3 (connecting to the other area 3 router) it will send area ID of 3 in its hello packet and this will match the area ID configured on the other router. Or the router could be configured with its physical interfaces in area 0. This means that the remote routers would also configure their interfaces that connect to this router in area 0. This would make the remote routers to be ABRs. While both of these alternatives are valid, I believe that most of us would prefer the first alternative in which this router is the ABR.


HTH


Rick

Kevin Dorrell Fri, 10/26/2007 - 06:56
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Rick,


Thanks for that explanation.


I was doing a lab exercise the other day in which there was a strange topology with two routers and three non-zero areas: one on the loopback of each router, and another on the link between the routers. And not an area 0 in sight.


The student had to work out that the way to get this topology to work was to create a virtual link between the two routers - in other words a virtual area 0 - transiting the area between the two routers.


Kevin Dorrell

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Richard Burts Fri, 10/26/2007 - 07:06
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Kevin


Indeed it is an interesting topology - and an interesting lesson about OSPF (or at least the Cisco implementation of OSPF). I have talked to many people who believe that the definition of Border Router is that the ABR is a router with interfaces in 2 different areas. In fact to really be an ABR the router must have interfaces in at least 2 areas and one of the areas must be area 0. In the case where the router has interfaces in 2 areas but not in area 0 (for example it has interfaces in area 2 and in area 3) it will maintain a Link State Data Base for each area but it will not advertise into area 3 any LSAs from area 2. It will only advertise into remote areas when it has an area 0. And you are right that configuring the virtual link creates a virtual area 0. Very good point :)


HTH


Rick

bvsnarayana03 Fri, 10/26/2007 - 07:50
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Thanks Kevin & Rick.


Great explanations with scenarios. Clarified lot many concepts.

Kevin Dorrell Fri, 10/26/2007 - 06:12
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Yes, that's fine too - it makes it quite recognisable when you view it from another router. But I would hard code it rather than let it pick it up automatically.


Kevin Dorrell

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