Please view attachment. I currently have an edge router in area 4 that is connected to the ABR (router A). Link 1 from the edge router to router A is configured for area 4. In the next year, we plan to tie another connection from the edge router in area 4 to an ABR in area 1, which is represented by the dotted line. Question: Do I configure link 2 to be in area 0? Do I configure link 2 to be in area 4? Do I configure link 2 to be in area 1? What is the best practice and recommendation? Thanks in advance.
Thank you for your perspective. In retrospect I will back off from my statement that none of the alternatives are attractive. I do believe that configuring the new link in either area 1 or in area 0 is problematic. But configuring the new link in area 4 does just extend area 4 and does provide an additional ABR for area 4.
I think you are both talking about the same, just using a slightly different terminology that leads to confusions, and I tend to agree with both of you.
In my opinion, the router to which a new link is being added is currently in the Area 4 and if the network is to be extended such that a new link is connected to another ABR, there is no problem with that. The Area 4 will simply have more ABRs, providing redundancy and possibly load balancing.
The added link, in my opinion, should be definitely placed into the Area 4 as well. Actually, we should forget about the Area 1 completely. ABRs in OSPF are not tied to particular non-backbone areas. As the topology change here involves only a router in the Area 4, let's talk only about Area 4 and backbone. We are simply extending the Area 4 to have another border router - in my opinion, it's that simple
I take exception with your statement that "There is nothing wrong with an non-area 0 router developing adjacencies with two different area 0 routers". If the edge router (currently a member of only a single are) develops adjacencies with ABRs in area 4 and area 1, then it suggests that it functions as an ABR for both areas. But in the Cisco implementation of OSPF it becomes a member of both areas but it is NOT an ABR for either router.
I do not believe that it is a common practice for a router to become a member of 2 different areas, but not to be an ABR for either area.
There is nothing wrong with an non-area 0 router developing adjacencies with two different area 0 routers. If this were a problem how could OSPF be a fault tolerant routing protocol? In today’s highly available networks this is a common practice.
In the diagram provided there will not be problems given any fault scenario. Careful though must be given to how to grow this network in the future; but this can be said for any network.
I believe that the answer about what is the best practice would be "do not do this".
Your "edge router" currently participates in a single OSPF area (and that keeps thing simple). If you do install the new link you have a couple of options.
- you can configure the link to be part of area 4 (configured this way on both routers). This keeps your edge router as a member of a single area. And it makes the ABR of area 1 to become an ABR of both area 1 and area 4. (the implications of this are not clear based on the limited information that we currently have).
- you can configure the link to be part of area 1. (configured this way on both routers) In this situation your edge router is a member of area 1 and of area 4. But it is NOT an ABR and it will not share routes between area 1 and area 4.
- you can configure the link as part of area 0 (configured on both routers). This will make your edge router to function as an ABR. But it will only know routes for area 1 and 4. It is possible that some router in one of those areas will forward to the edge router (as an ABR) assuming that it has knowledge of all areas but the router will not have knowledge and will drop the packet.
In my opinion none of these alternatives are attractive.
Area 4 Edge router has to participate 2 areas. One is area 4(Link 1 interface) and another one is area 1(Link 2 interface).
Based on the link bandwidth details Area 4 Edge rotuer will decide the primary path.
Extend area 4 to the ABR servicing area 1 and you'll be good to go in any failure situation and still keep to the basic tenet of OSPF that all area's must be adjacent to area 0.