What is the best practice in conserving router resources? Should a single ABR router be the DR for all areas? Will that have a negative impact on the resources, or would it be better to let other routers act as the BR?
2 Answers
In OSPF does a DR consume more resources than a BDR?
Both the DR and BDR for a link will form a full adjacency with any other OSPF routers on the same link. That is what consumes the most router resources.
Should a single ABR router be the DR for all areas?
There is a problem with that idea. DR/BDR is per (broadcast/non-broadcast) link, not area. Each area may require multiple DRs/BDRs because there are multiple broadcast links in an area. You may also have entire areas that consist of point-to-point links, where there are no DRs in the entire area.
Assume a network as in this diagram, where all the links are ethernet (broadcast) links:
The ABR can certainly be the DR for all the orange links (it needs to have enough resources to handle being the DR for all those links), but each of the red links will also need a DR, and the ABR cannot be the DR for those links.
In fact, there is no single router in an area that can be the DR for all the red links in the area because no single router in any area is connected to all the red links in the area.
Also, for example, if all the links in the yellow area are point-to-point links, including the links to the ABR, then the yellow area will not have any DRs, and the ABR will not be a DR for the yellow area.
If all the links in the AS are point-to-point links, then there will be no DRs in the entire AS.
In an OSPF AS that has multiple areas, you must have at least one ABR, but DRs have nothing to do with an area. DRs are used on broadcast/non-broadcast links so that not every router on the link needs to form a full adjacency with every other router on the link, only with the DR and BDR for that link.
I think you are conflating two different concepts.
answered Feb 16, 2019 at 1:55
Designated router (DR) selection: In general, the DR and backup designated router (BDR) on a multiaccess link (for example, Ethernet) have the most OSPF work to do. It is a good idea to select routers that are not already heavily loaded with CPU-intensive activities to be the DR and BDR. In addition, it is generally not a good idea to select the same router to be the DR on many multiaccess links simultaneously.
http://www.ciscopress.com/articles/article.asp?p=1763921&seqNum=6