Cisco Certified Support Technician (CCST) NetworkingIP ConnectivityMedium
A network administrator is troubleshooting an OSPF adjacency issue between two routers. They observe that the Designated Router (DR) and Backup Designated Router (BDR) are not being elected on a multi-access segment. Which of the following interface types is typically associated with OSPF DR/BDR election?
- ALoopback
- BBroadcast multi-access
- CPoint-to-point
- DPoint-to-multipoint
Show answer & explanationAnswer & explanation
Correct answer: B. Broadcast multi-access
DR/BDR elections are specific to OSPF on broadcast multi-access network types (like Ethernet LANs). On these segments, a DR and BDR are elected to reduce the number of adjacencies and the amount of routing information exchanged, thereby optimizing the OSPF process.
Why the other options are wrong
- A. Loopback interfaces are always considered OSPF passive interfaces and are used to determine the router ID; they do not participate in DR/BDR elections.
- C. Point-to-point networks (e.g., serial links) do not elect DR/BDRs because there are only two routers, thus no need for a central point of contact.
- D. Point-to-multipoint networks do not typically elect DR/BDRs, though variations exist.
OSPF DR/BDR Election
In OSPF, a Designated Router (DR) and a Backup Designated Router (BDR) are elected on broadcast multi-access network segments to centralize the exchange of Link-State Advertisements (LSAs) and reduce the number of adjacencies.
- Occurs only on broadcast multi-access networks (e.g., Ethernet).
- DR and BDR manage LSA flooding.
- Reduces the number of OSPF adjacencies from N*(N-1)/2 to N-1.
Memory trick: Many choices need a leader and a backup.