OSPF RFC describes two different modes of operation of OSPF for (1) point-to-point and (2) broadcast networks (and others).
The operation on (2) involves selecting designated router and backup designated routers, and consequences thereof. Let's call it DR/BDR mode. AFAIK OSPF should be configured with the type for each interface.
If several routers are connected by a switch it makes sense that DR/BDR mode happens.
Recently, I have watched a cisco certification related video: (in particular question at this timestamp and this timestamp). In the video, there was a DR/BDR election on an Ethernet link, which was marked as broadcast, but the link was connecting only two routers? It seems really strange that one wants to have a link between only two routers work as broadcast link.
Does OSPF have to work in DR/BDR mode on Ethernet? Is it rather default configuration of cisco routers that is supposed to be overwritten. Is it bad configuration to still use Ethernet interface as broadcast? Or is it just a certification question that has no relevance in practice.
I do understand that multicast addresses in packets make no difference. I am more interested in the added overhead of broadcast network:
- for broadcast networks, OSPF creates so called network LSAs. This should mean that an extra LSA must be disseminated, stored in the database, and shortest-path calculation (dijkstra) gets an extra node and 2 links to consider.
- flooding procedure. does having DR and BDR cause extra steps in LSA dissemination? (if I understand the procedure correctly, LSAs from DR do not cause anything extra, but an LSA from BDR should be re advertised back on the link, thus it is sent twice.)
- how do routers know whether to put network from network LSA in routing table? this network should be marked as transit network (because it has 2+ routers attached and can be used for transit). But neither of the routers knows whether this network has end-systems or not. If it has end-systems, then prefix of these end-systems needs to be in the forwarding table. How does OSPF know whether to do it or not? Does it per-default assume that there are no end-systems on transit networks, or does one need to configure this separately?.
Does this all cause so little overhead for modern routers (with their processing power) that it does not matter at all?