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Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

A network administrator executes the following configuration commands on a Cisco router interface connected to an Ethernet switch:

text
R1(config)# interface GigabitEthernet0/1
R1(config-if)# ip ospf network point-to-point

Prior to this change, GigabitEthernet0/1 was operating under the default Cisco IOS OSPF settings for Ethernet interfaces. Which statement accurately describes the effect of this configuration change on OSPF DR/BDR elections and timer values for GigabitEthernet0/1?

  1. OSPF DR and BDR elections are suppressed on the interface, while the default Hello timer remains 10 seconds.Answer
  2. B
    The interface priority is automatically set to 0, causing the router to withdraw from DR/BDR elections while increasing the Hello timer to 30 seconds.
  3. C
    The default OSPF Hello timer automatically changes from 10 seconds to 30 seconds, though DR and BDR elections continue as normal.
  4. D
    DR and BDR elections are still performed, but R1 automatically forces itself to become the Designated Router on the segment.

Answer

OSPF DR and BDR elections are suppressed on the interface, while the default Hello timer remains 10 seconds.
On Cisco routers, Ethernet interfaces default to the Broadcast network type, which elects a Designated Router (DR) and Backup Designated Router (BDR) and uses a 10-second Hello interval. Manually overriding the network type with `ip ospf network point-to-point` instructs OSPF to treat the link as a direct point-to-point connection. As a result, DR and BDR elections are completely suppressed on that interface. Because the standard Hello interval for OSPF Point-to-Point interfaces is also 10 seconds, the Hello timer remains unchanged.

Step-by-Step Solution

1
Identify default OSPF parameters for Ethernet media.
By default, Cisco IOS assigns the OSPF network type 'Broadcast' to Ethernet interfaces, which uses a 10-second Hello timer (40-second Dead timer) and elects a DR/BDR.
Ethernet is multi-access media, requiring DR/BDR to reduce adjacency count.
2
Analyze the impact of the 'ip ospf network point-to-point' command.
Configuring 'point-to-point' changes the network type to Point-to-Point, which does NOT elect a DR/BDR on the interface.
Point-to-Point networks assume only two routers exist on the link, rendering designated routers unnecessary.
3
Determine timer adjustments associated with the Point-to-Point network type.
The Point-to-Point network type in OSPFv2 defaults to a 10-second Hello timer and 40-second Dead timer.
Both Broadcast and Point-to-Point OSPF network types share the same default 10-second Hello / 40-second Dead timer interval.

Key Concept

OSPFv2 Network Types and DR/BDR Suppression
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Topics

OSPFv2 Network Types and DR/BDR Selection

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