OSPF network types — where is a Backup Designated Router (BDR) elected? Identify the OSPF network types that perform DR/BDR elections on multiaccess segments.

Introduction / Context:
On multiaccess networks, OSPF elects a Designated Router (DR) and Backup DR (BDR) to reduce adjacency count and LSA flooding. Knowing which network types trigger these elections is crucial to predicting neighbor behavior and LSA flow.

Given Data / Assumptions:

• OSPF recognizes several network types: broadcast multi-access (for example, Ethernet), non-broadcast multi-access (NBMA, for example, Frame Relay), and point-to-point.
• DR/BDR election occurs only on multiaccess segments.

Concept / Approach:
Broadcast multi-access and NBMA networks are multiaccess; they can connect multiple routers to the same segment. To reduce n*(n−1)/2 adjacency complexity, OSPF elects a DR and BDR on such segments. On point-to-point links (two routers only), there is no election because only a single adjacency exists, and the overhead of elections is unnecessary.

Step-by-Step Solution:
Classify network types: broadcast and NBMA are multiaccess; point-to-point is not.Apply OSPF behavior: DR/BDR elections occur on multiaccess networks.Select broadcast multi-access and NBMA as the correct types.

Verification / Alternative check:
Running show ip ospf interface on Ethernet (broadcast) or Frame Relay (NBMA) shows DR/BDR roles; on point-to-point serial links no DR/BDR is elected.

Why Other Options Are Wrong:

• B/C: Point-to-point links do not run DR/BDR elections.
• D: False; OSPF often elects DR/BDR on multiaccess networks.
• E: Incomplete; broadcast multi-access is also included.

Common Pitfalls:
Assuming any Ethernet link is point-to-point; switched Ethernet is broadcast multi-access from OSPF’s perspective and elects DR/BDR.

Final Answer:
Broadcast multi-access and Non-broadcast multi-access