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Difficulty: HardOSPFv2 Network Types and DR/BDR Selection

A network engineer changes the OSPFv2 network type on a GigabitEthernet interface linking two Cisco routers from the default Broadcast type to Point-to-Point using the interface subcommand `ip ospf network point-to-point`.

Which two operational changes occur on this interface as a direct result of this modification? (Choose two.)

  1. OSPF bypasses Designated Router (DR) and Backup Designated Router (BDR) elections on the interface segment.Answer
  2. The interface switches from 224.0.0.6 to exclusively using 224.0.0.5 for all OSPF multicast communication.Answer
  3. C
    The default OSPF Hello timer automatically increases from 10 seconds to 30 seconds on the interface.
  4. D
    The OSPF interface priority is automatically rewritten to 0 in the running configuration.
  5. E
    Dynamic neighbor discovery is disabled, requiring explicit configuration of the OSPF neighbor command.

Answer

The two correct statements are that OSPF bypasses DR/BDR elections on the interface segment, and all OSPF multicast communication uses 224.0.0.5 instead of 224.0.0.6.
Configuring `ip ospf network point-to-point` on an Ethernet interface alters OSPF behavior so that DR/BDR elections are completely bypassed. Additionally, because there is no DR or BDR to listen on 224.0.0.6, all OSPF traffic on the link is directed to the All-OSPF-Routers multicast address 224.0.0.5.

Step-by-Step Solution

1
Analyze DR/BDR election requirements for OSPF network types
Broadcast networks elect a DR and BDR to reduce adjacency overhead on multiaccess media. Point-to-Point networks connect exactly two routers, eliminating the need for DR/BDR roles.
On point-to-point links, routers establish a direct FULL adjacency with each other without intermediate election logic.
2
Evaluate multicast address usage across OSPF roles
On multiaccess broadcast networks, DROther routers send Link State Updates to 224.0.0.6 (AllDRouters), and the DR floods updates to 224.0.0.5 (AllSPFRouters). On Point-to-Point networks without a DR, all OSPF packets (Hellos, LSU, LSAck) are sent directly to 224.0.0.5.
Since 224.0.0.6 is reserved exclusively for communicating with Designated Routers, it is not used on point-to-point interfaces.
3
Verify timer and neighbor discovery behaviors for Point-to-Point media
Point-to-Point interfaces maintain 10-second Hello and 40-second Dead default timers (same as Broadcast) and continue using multicast for dynamic neighbor discovery.
Longer 30-second timers and manual static neighbor commands are characteristics of Non-Broadcast Multi-Access (NBMA) networks, not Point-to-Point networks.

Key Concept

OSPFv2 Network Types and DR/BDR Selection Traits
Estimated Time:2m 0s
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Topics

OSPFv2 Network Types and DR/BDR Selection

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