Question

Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

Match each OSPFv2 network type or interface state on the left with its correct DR/BDR election requirement and default timer characteristics on the right.

  • Broadcast Multiaccess interface with default interface settingsElects a DR and BDR; default Hello and Dead timers are 10s and 40s
  • Point-to-Point interface over a serial link running HDLCDoes not elect a DR/BDR; default Hello and Dead timers are 10s and 40s
  • Ethernet interface configured with the command `ip ospf priority 0`Ineligible for DR/BDR election (remains DROTHER); Hello/Dead timers remain 10s and 40s
  • Point-to-Multipoint interface environmentDoes not elect a DR/BDR; default Hello and Dead timers are 30s and 120s

Answer

Broadcast Multiaccess maps to electing DR/BDR with 10s/40s timers; Point-to-Point maps to no DR/BDR with 10s/40s timers; Interface priority 0 maps to election ineligibility (DROTHER); Point-to-Multipoint maps to no DR/BDR with 30s/120s timers.
Each OSPF interface network type specifies whether DR/BDR elections are held and sets default Hello/Dead timers. Broadcast Ethernet networks elect DR/BDR with 10s/40s timers. Point-to-Point links bypass DR/BDR elections with 10s/40s timers. Setting priority to 0 on a broadcast interface retains standard timers but prevents DR/BDR election. Point-to-Multipoint skips DR/BDR elections and uses 30s/120s default timers.

Step-by-Step Solution

1
Analyze OSPF election requirements per network type
Broadcast networks elect DR/BDR; Point-to-Point and Point-to-Multipoint networks do not elect DR/BDR.
DR/BDR election is only required on multiaccess media (Broadcast and Non-Broadcast) to reduce link-state packet duplication.
2
Determine interface priority impact
Setting priority to 0 makes a router ineligible to become DR or BDR regardless of router ID.
OSPF interface priority ranges from 0 to 255, where 0 explicitly prevents candidacy.
3
Evaluate default Hello and Dead timers for each network type
Broadcast and Point-to-Point default to 10s/40s, while Point-to-Multipoint defaults to 30s/120s.
Non-broadcast and point-to-multipoint OSPF network types assume slower WAN links and use 30-second Hello / 120-second Dead timer defaults.

Key Concept

OSPFv2 DR/BDR election rules and default timer intervals vary according to the interface network type and priority setting.
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