Why does OSPF elect a DR and BDR, and how is the election decided?
Show the answer
Say this first
On a multi-access segment with n routers, a full mesh of adjacencies would be n(n-1)/2 and every change would flood n times. Instead every router goes Full only with the DR and BDR, and the DR originates one Type 2 LSA for the segment. Highest priority wins (default 1, 0 means never), then highest router ID.
Go deeper
The rules
- Runs on broadcast and NBMA network types only. Not on point-to-point or point-to-multipoint.
- Priority is per interface, 0 to 255, default 1. Priority 0 means the router can never be DR or BDR.
- Tie on priority: highest router ID wins.
- The BDR is elected first, then the DR. If there is no DR, the BDR is promoted.
- A router that comes up on a segment waits the wait timer (equal to the dead interval, 40 seconds) before it elects, so it can learn about an existing DR first.
Influencing it
interface GigabitEthernet0/0/0
ip ospf priority 255On the routers that should never be DR, such as small branch routers on a shared WAN segment,
I set ip ospf priority 0.
What you see
R3# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 255 FULL/DR 00:00:36 10.0.123.1 GigabitEthernet0/0/0
2.2.2.2 1 FULL/BDR 00:00:39 10.0.123.2 GigabitEthernet0/0/0
4.4.4.4 1 2WAY/DROTHER 00:00:33 10.0.123.4 GigabitEthernet0/0/0R3 is a DROTHER. It is Full with the DR and BDR and stays 2WAY with R4, the other DROTHER.
They will ask next
Is the DR elected per area or per router?
Per segment, meaning per multi-access interface. A router can be DR on one interface and DROTHER on another.
What does the DR advertise that others do not?
The Type 2 network LSA for the segment. It lists the attached routers and the subnet mask. Its link state ID is the DR's interface address.
Traps
- Saying the highest IP address wins. It is highest priority, then highest router ID.
- Saying the DR is the "router with the most routes". The DR only matters for flooding on that segment.