In some networks that are not point to point, such as Ethernet LANs and Frame Relay, there can be more than two routers that can exist in a single segment.
Without any control system in place, scalability issues could arise with the more routers that partake in OSPF on the subnet – more network traffic, more CPU usage, more memory usage, larger processing times.
OSPF has a built in mechanism to help keep multiple neighbouring routers manageable. There is an election that nominates a router to become a designated router (DR) on the subnet. The designated router becomes the central node that manages the adjacency states of all other neighbouring routers on that sub-network.
The designated router is responsible for providing updates for all OSPF routers on that sub-network too.
During the election of a designated router, there is an election to nominate a backup designated router (BDR). The backup router is there in the event of the designated router going offline, providing continuity in the OSPF process without the need to flood new adjacencies and relearning of routes. The BDR forms adjacencies with all routers on the network subnet along with the DR.
During the 2-way formation, if a hello packet is received with an address other than 0.0.0.0 for the DR or BDR, the router will assume that those addresses are the actual DR and BDR without the need for an election.
If a router has a priority configured between 1 and 255 on its interface, it will attempt to become the DR. By default, all interfaces have a priority of 1. The routers place their RID and OSPF priorities in their Hello packets.
When sending packets with their RID and OSPF priorities, if a router deems itself to be the most favourable router, it will continue to send packets with its own RID and priority. If a more favourable router is identified, it will send its RID and priority in the DR field instead.
When a priority is the same, the highest RID will be used instead.
The OSPF priority is set with the command ip ospf priority in the interface configuration. If the priority is 0, the router will not take part in the DR/BDR election.
When an OSPF router learns of a new route in this shared network subnet, it sends an link state advertisment (LSA) to the 224.0.0.6 multicast address.
This 224.0.0.6 multicast address is only listened to by the DR and BDR routers.
The DR will send an acknowledgement back to the originating router by a unicast address, before flooding the LSA to all other members via the multicast address 224.0.0.5
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