CompTIA Network+ (N10-009)Network ImplementationMedium
Three OSPF routers are connected to the same Ethernet multiaccess segment. Which mechanism does OSPF use on this segment to reduce the number of adjacencies formed and minimize routing update traffic?
- AUse of passive interfaces
- BElection of a Designated Router (DR) and Backup Designated Router (BDR)
- CManual adjustment of interface cost
- DConfiguration of route reflectors
Show answer & explanationAnswer & explanation
Correct answer: B. Election of a Designated Router (DR) and Backup Designated Router (BDR)
On multiaccess networks, OSPF elects a DR and BDR so that other routers form full adjacencies only with the DR/BDR instead of with every neighbor, drastically reducing the number of adjacencies and the volume of link-state advertisements exchanged.
Why the other options are wrong
- A. Passive interfaces stop OSPF hellos on an interface entirely, preventing adjacencies rather than reducing their number efficiently.
- C. Interface cost affects path selection, not the number of adjacencies formed.
- D. Route reflectors are a BGP concept used to avoid full iBGP mesh, not an OSPF feature.
OSPF DR/BDR Election
On multiaccess (broadcast) networks, OSPF elects a Designated Router and Backup Designated Router so all other routers only peer with them, reducing adjacency count.
- Election based on OSPF priority, then highest Router ID
- DROTHER routers don't form full adjacency with each other
- Not needed on point-to-point links
Memory trick: DR is the 'hub' everyone talks to instead of gossiping with everyone.