Cisco Certified Support Technician (CCST) NetworkingIP ConnectivityMedium
A network engineer is configuring OSPF on a broadcast multi-access network segment. Two routers, R1 and R2, are connected to this segment. R1 has a router ID of 192.168.1.1, and R2 has a router ID of 192.168.1.2. Both routers have their interface priorities set to the default value. Which router will be elected as the Designated Router (DR)?
- ANeither, as both have default priorities and will become DRothers.
- BR1, because it was powered on first.
- CR1, because its router ID is numerically lower.
- DR2, because it has the higher router ID.
Show answer & explanationAnswer & explanation
Correct answer: D. R2, because it has the higher router ID.
In OSPF DR/BDR election, if interface priorities are equal (default is 1), the router with the highest router ID becomes the DR. Since R2 has a router ID of 192.168.1.2, which is higher than R1's 192.168.1.1, R2 will be elected as the DR.
Why the other options are wrong
- A. A DR will always be elected on a broadcast multi-access segment unless there are no OSPF-enabled routers.
- B. Power-on order is not a factor in DR/BDR election unless all other factors are equal and it's a tie-breaker (which is not directly part of the standard election process).
- C. A lower router ID does not win the DR election; the highest router ID wins when priorities are equal.
OSPF DR/BDR Election
OSPF elects a Designated Router (DR) and a Backup Designated Router (BDR) on broadcast multi-access networks to reduce redundant adjacencies and manage LSA flooding.
- Highest OSPF interface priority wins (0-255, default 1).
- If priorities are equal, highest Router ID wins.
- Router ID is highest loopback IP, or highest active interface IP if no loopback.
- Election is non-preemptive.
Memory trick: Priority is king, but Router ID breaks a tie.