On a multi-access OSPF network such as Ethernet, how is the Designated Router (DR) chosen among the routers on that segment?

Introduction / Context:
This question tests your understanding of how OSPF elects a Designated Router and Backup Designated Router on broadcast or multi-access networks such as Ethernet. DR and BDR roles reduce the amount of neighbor relationships and LSDB flooding required, improving scalability. Knowing the election rules helps you predict and control which router will take the DR role in a given design.

Given Data / Assumptions:
- The network type is multi-access (for example, Ethernet) where multiple OSPF routers share the same segment.
- OSPF must elect a DR and a BDR on this segment.
- Interface OSPF priorities and router IDs are correctly configured or defaulted.
- The question asks for the deterministic rule by which the DR is chosen.

Concept / Approach:
On broadcast and non-broadcast multi-access networks, OSPF elects a DR and a BDR. Each router interface has an OSPF priority value, typically defaulting to 1, and can be set to 0 to prevent the router from becoming DR or BDR. The router with the highest OSPF priority on the interface wins the DR election. If multiple routers share the same highest priority, the router with the highest OSPF router ID is selected as the DR among them. The BDR is chosen using the same criteria from the remaining candidates.

Step-by-Step Solution:
Step 1: Recall that each OSPF-enabled interface on a multi-access network has a configurable priority value.Step 2: Understand that only routers with a nonzero priority (greater than 0) are eligible to become DR or BDR.Step 3: Determine that the router with the highest interface priority is chosen as the DR candidate.Step 4: If two or more routers tie for the highest priority, compare their OSPF router IDs.Step 5: Select the router with the highest router ID among those with the top priority as the DR; the next best becomes the BDR.

Verification / Alternative check:
In a lab, you can configure different ip ospf priority values on router interfaces and observe which one becomes DR after the election. If you set one router to priority 0 and another to a higher value, the zero-priority router will never become DR. If all priorities are equal, changing the router ID on one of the routers to a higher value will cause it to win the election upon reset, confirming the tie-breaking behavior.

Why Other Options Are Wrong:
Option A incorrectly bases the election on interface cost to the backbone, which is not part of the DR algorithm. Option B suggests that the first booted router remains DR forever, ignoring the fact that elections can occur again and that priority and router ID drive the decision. Option D bases the election on neighbor count, which is more relevant to some other protocols but not OSPF DR selection. Option E implies that DR selection is purely manual, which is not the normal behavior; administrators influence elections indirectly via priority and router ID instead.

Common Pitfalls:
Many learners assume that the lowest router ID wins, because lower metrics are usually better in routing decisions. In OSPF DR elections, however, the highest router ID wins when priorities tie. Another pitfall is forgetting that DR and BDR roles do not preempt automatically when a higher-priority router joins after an election; you must clear adjacencies or reset routers to force a new election in many cases. Understanding these rules helps avoid surprises in production networks.

Final Answer:
OSPF chooses the router with the highest interface priority as the DR on a multi-access network; if there is a tie, the router with the highest router ID among those routers becomes the DR.