Designated Router Note

In a multi-access environment, such as a LAN/MAN(and sometimes ethernet WAN), make OSPF less chatty, and to lower the CPU load due to inter-neighbor communications, OSPF will hold an election for a Designated Router(DR) and a Backup Designated Router(BDR). Once this election has occurred, all routing updates will be forwarded from the "DROTHER" routers(that is the term used for non-DR/BDR routers) to the DR/BDR. The DR and BDR maintain an equal picture of the topology due to this. The DR will then update it's LSAs and will propogate any required changes out to the rest of the DROTHER routers.

The election is based on the Router Priority (default 1) and Router ID. If your priority is zero then you will not be elected.

The DR and BDR do more or less the same thing, but the BDR will not send out updates unless it sees the DR go down. If the BDR sees the DR go down, it will promote itself to DR and assume that list of responsibilities.

First, routers that are in the same segment (such as an ethernet network) become neighbors in that segment. Note that the routers interfaces connected to that segment must belong to the same area.


In addition to the neighborship, there is also adjacency. Aadjacent routers are those who exchange the route database. So in order to minimize the information being exchanged in one segment, DR and BDR are elected. Then, adjacency is only formed between two routers when one of them is the DR or the BDR. Hence, a DROTHER sends the LSA information to only its adjacent routers which are the DR and the BDR, the DR then informs all the other ones -which all are in fact adjacent to the DR.  DROTHER represents routers of non DR or BDR routers. 

224.0.0.6 in a DR-based environment is used to pass information from any router to the DR/BDR. (AllDR Routers)


224.0.0.5 in a DR-based environment is used to pass information about valid routes FROM the DR/BDR to other routers. (AllSPF Routers)

DROTHERs don't flood the network with anything - they multicast updates to the DR/BDR multicast addresses - which are not monitored by the other DROTHER routers on the segment.

 Once the DR processes the updated LSA information, it will repackage it and multicast the update out to all of the routers it is peered with - including the BDR. I do believe BDR looks for these communications from the DR as part of the keepalive process.

The DR is elected by the Hello Protocol. A router's Hello packet contains its Router Priority, which is configurable on a per-interface basis. When a router's interface becomes functional, it checks to see whether there is currently a DR. If there is, it accepts that DR, regardless of its Router Priority. Otherwise, it becomes itself DR if it has the highest Router Priority on the network.

The DR is the endpoint of many adjacencies. To optimize the flooding procedure, the DR multicast its Link State Update packets to the address AllSPFRouters, rather than sending separate packets over each adjacency.

Router nodes are labelled with their Router ID. Transit networks are labelled with the IP address of their DR. Then, when the DR changes, this will cause the network and all its routers to originate new LSAs. Until the Link-State Database converges, some temporary loss of connectivity occur. This may result in ICMP unreachable messages being sent in response to data traffic. For that reason, the DR should change only infrequently. Router Priority should be configured to elect the most reliable router on a network as DR.