The original post asks 2 questions and I offer these answers:
1) how can there be more than 2 routers on one link? It depends on how you interpret the term link. In modern switched technology networks we might consider each switch interface to be a separate link. But in the older technology where the term link was used it was quite different. Think of a 10BaseT network. There was a cable that provided the Ethernet network. Multiple devices connected to the Ethernet cable. And the link was the cable which could easily have more than 2 routers connected to it. When we use the term link in the context of OSPF routers we are thinking more of the vlan broadcast domain than we are of the collision domain.
2) how a priority on one port can determine the DR / BDR. I would say first that priority can influence the election but not necessarily determine the election. In the OSPF election for BDR and DR the BDR election takes place first so lets start with a simplified explanation of the BDR election. The election takes place on multi access interfaces (other type of interfaces such as point to point or loopback do not have elections). Each OSPF interface has a priority. The default value is 1 and if we want to influence the election we can configure a different priority. For some routers we may want that they NOT be elected and we configure their priority to be zero. Other routers we may want them to be elected and we can configure their priority to a higher value.
When a router joins a network it sends a Hello message, and that message has fields that identify the BDR and the DR. The router listens for Hello messages from other routers on the network. If it receives Hello messages from another OSPF router it looks in that Hello message at the BDR and DR fields. If the BDR field is non zero then there has already been an election and the new router knows the identity of the BDR (and of the DR) and there is not a new election (even if the new router has a priority higher than the current BDR it does not cause a new election). If the BDR field is zero then there are at least 2 routers on the network and an election takes place. The election process compares the priority of the eligible routers. If there is a single router with priority higher than other routers then this router wins the election and becomes BDR. If there are multiple routers with the highest priority then the election compares the RID of the candidates and chooses the BDR based on the RID.
Once the BDR is settled the routers consider the DR. If the DR field in the received Hello message is non zero then a DR has already been elected and there is no new election. If the DR field is zero then there is not an existing DR and the election process for DR is to promote the BDR to becomes DR.
Just because we configure a higher priority on an OSPF interface does not necessarily mean that the router is going to be elected. Consider these possibilities:
- if a router joins the network and has configured a high priority but the election has already taken place then the new (high priority) router will not become DR (or VDR).
- if a router joins the network and has configured a high priority and there is another router with the same high priority then the election will be based on RID and not priority.
One other thing to consider. If we assign a priority of zero then the router will never will an election. What if all the routers on the link have priority of zero? Remember that OSPF routers on a multi access network will form full adjacency only with the DR and with the BDR. If there is no DR and no BDR then no adjacencies are formed and therefore no OSPF routes are exchanged (and you basically have a broken network).
HTH
Rick
