OSPF DR/BDR Election explained

Lessons Discussion
62

Azeem,
The BDR is elected first (see step #7 below). Here is a break down of what happens during the election:

  1. Once an OSPF becomes active on a multi-access network, it sets the DR and BDR values to 0.0.0.0 which indicates these are unknown. It also starts a wait timer of the value of the dead interval.
  2. The router starts neighbor discovery. It sends the 0.0.0.0 values for DR/BDR
  3. If a received Hello includes values for DR/BDR, these are accepted and the wait timer is stopped
  4. If the wait timer expires, the DR election starts
  5. A list of neighbors on the multiaccess network is established that are eligible for being DR/BDR (their priority is higher than zero)
  6. A subset of routers from this list is created. This subset excludes routers listing their own address as the DR (so routers claiming to be the DR are excluded)
  7. From this subset, select the router with the highest priority/RID/Loopback/Address to be the BDR
  8. From the original (not subset list), select the Router with the highest priority/RID/loopback/address to be the DR
  9. If there are no routers originally claiming to be the DR, then promote the chosen BDR to a DR, and elect another BDR via the same method
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