DR selection between Area 0 and other areas

I am Confused about DR Selection. If DR is not Area Specific and I have three areas, I would like to keep DR area 0 in the DATA Center. Will all other areas build against that Area 0 Router?

We need to be careful with the language here. The confusion is the thought that DRs are not area specific. Perhaps what you mean is that there is not necessarily a 1:1 ratio between DRs and Areas. That is a true statement, because a particular area could have zero, one, or multiple DRs. This is because DRs are one-to-one with multi-access networks (that are a single subnet).

Where our thinking might lead us astray, however, is to make the leap to the false statement “a DR can have ‘clients’ from multiple different areas.” In OSPF terms (which is the opposite in IS-IS, by the way), the networks live in areas, NOT the routers. Of course, an ABR is an example of this. This means that it cannot be the case that a DR can “extend” beyond an area.

So think of it this way: Areas can have any number of DRs, but DRs can have at most one area. In light of this understanding, I think you will see that having “THE DR” in Area 0 and have other areas build against it is impossible.

*Edit*: Technically, it is possible for the same physical router to be the DR in multiple areas. This could be accomplished by having the router attached to multiple multi-access networks, each of which is in a different area. However, even in this case, these would be considered different DRs because each has different “clients” that would receive the multicast from the DR in question.

Thank you for your quick reply. It is very clear now :slight_smile: