Lesson 41: OSPF DR/BDR Election

On Multi-Access Networks (like a standard Ethernet segment connected to a switch with multiple routers), OSPF faces a challenge: every router would try to form an adjacency with every other router ($N(N-1)/2$ adjacencies).

Designated Router (DR) and Backup Designated Router (BDR)

To reduce complexity, OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR) on each multi-access segment.

• The DR acts as the central point for LSA exchange.
• All non-DR/BDR routers (DROthers) form FULL adjacency only with the DR and BDR.

DR/BDR Election Process

The election is based on the following criteria, in order:

1. Highest OSPF Priority: Configured per interface (default is 1). Priority 0 prevents the router from ever becoming DR/BDR.
2. Highest Router ID (RID): The highest RID wins if priorities are tied.

Configuration (Interface Mode):

ios Router(config-if)# ip ospf priority 100

The election is non-preemptive; if a new router with a higher priority joins, the current DR/BDR remain unless one fails.