Question

Difficulty: HardOSPFv2 Network Types and DR/BDR Selection

Match each OSPFv2 network type configuration on the left with its corresponding DR/BDR election requirement and default Hello/Dead timer characteristics on the right.

  • Broadcast (`ip ospf network broadcast`)Elects a DR/BDR automatically using multicast Hello packets; default Hello timer is 10 seconds and Dead timer is 40 seconds.
  • Point-to-Point (`ip ospf network point-to-point`)Does not elect a DR/BDR; sends multicast Hello packets with default Hello timer of 10 seconds and Dead timer of 40 seconds.
  • Point-to-Multipoint (`ip ospf network point-to-multipoint`)Does not elect a DR/BDR; sends multicast Hello packets to all neighbors with default Hello timer of 30 seconds and Dead timer of 120 seconds.
  • Non-Broadcast (`ip ospf network non-broadcast`)Elects a DR/BDR but requires manual `neighbor` statements due to lack of multicast support; default Hello timer is 30 seconds and Dead timer is 120 seconds.

Answer

Broadcast pairs with DR/BDR elected automatically with 10s/40s timers. Point-to-Point pairs with no DR/BDR election and 10s/40s timers. Point-to-Multipoint pairs with no DR/BDR election and 30s/120s timers. Non-Broadcast pairs with DR/BDR elected using manual neighbor statements and 30s/120s timers.
Each OSPF network type has distinct rules regarding whether DR/BDR elections occur, whether multicast is used for neighbor discovery, and which default timers apply. LAN-oriented types (Broadcast and Point-to-Point) use 10-second Hello and 40-second Dead timers, while WAN-oriented types (Point-to-Multipoint and Non-Broadcast) use 30-second Hello and 120-second Dead timers. Multiaccess types (Broadcast and Non-Broadcast) perform DR/BDR elections, whereas point-to-point styles (Point-to-Point and Point-to-Multipoint) bypass DR/BDR elections entirely.

Step-by-Step Solution

1
Analyze election requirements for multiaccess vs. point-to-point network types.
Broadcast and Non-Broadcast networks require DR/BDR elections because multiple routers share the medium. Point-to-Point and Point-to-Multipoint networks do not elect DR/BDRs.
DR/BDR selection reduces adjacency formation overhead from n(n1)/2n(n-1)/2 to 2n32n-3 on multiaccess segments.
2
Analyze neighbor discovery mechanisms (multicast vs. unicast).
Broadcast, Point-to-Point, and Point-to-Multipoint use multicast (224.0.0.5224.0.0.5). Non-Broadcast multiaccess requires manual unicast neighbor definition via the `neighbor` command.
NBMA media do not natively support broadcast or multicast traffic.
3
Evaluate default OSPFv2 Hello and Dead timers for each network type.
LAN types (Broadcast, Point-to-Point) default to 10s Hello / 40s Dead. WAN types (Point-to-Multipoint, Non-Broadcast) default to 30s Hello / 120s Dead.
Slower WAN links require higher timer tolerances to avoid unnecessary neighbor flap state transitions.

Key Concept

OSPFv2 Network Types, DR/BDR Election Requirements, and Default Timers
Estimated Time:2m 0s
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