Lab ipc-07 - OSPF DR/BDR Election

On a broadcast OSPF segment, the protocol elects a Designated Router (DR) and a Backup DR (BDR). Other routers form adjacencies only with the DR/BDR rather than with every neighbor on the segment, dramatically reducing LSA flooding. The election rule is: highest OSPF priority wins; on a tie, highest router-id wins. Priority 0 means "never become DR/BDR." This lab walks through the election on the OSPF Reference Lab's Area 0 broadcast LAN, where R2 is explicitly forced to be DR with priority 100.

What you will learn

• How DR and BDR are elected on broadcast networks
• How to manually force which router becomes DR using ip ospf priority
• Why the election is non-preemptive (incumbents stay)
• How to read show ip ospf interface and show ip ospf neighbor detail for election state
• The DROTHER role and why it matters

What this lab does NOT cover

• DR/BDR on point-to-point links - there are no DR/BDRs on P2P (covered in ipc-06)
• Adjacency state machine details (EXSTART, EXCHANGE, LOADING)

Topology

The election rule

1. Highest OSPF priority wins.
2. On a tie, highest router-id wins.
3. Priority 0 = "never become DR or BDR" - automatically excluded.
4. Once elected, the DR and BDR stay even if a router with higher priority joins - non-preemptive.

The non-preemption part is critical. If you bring up a new router with priority 200 onto a segment where DR was already elected with priority 100, the new router does NOT take over. You would need to clear OSPF (cycle the router or use clear ip ospf process) for re-election.