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Difficulty: MediumHigh Availability and Redundancy Concepts

A network administrator configures a redundant gateway topology using a First Hop Redundancy Protocol (FHRP) to provide high availability for a corporate subnet. During a scheduled failover test, the primary router is powered off. Although logs confirm that the standby router successfully transitioned to the active role, all client workstations immediately lose outbound internet connectivity. Troubleshooting reveals that workstation traffic continues to be routed toward the MAC and IP addresses of the powered-off primary router interface. Which of the following configuration errors is the most likely cause of this issue?

  1. The DHCP scope is distributing the physical interface IP address of the primary router as the default gateway rather than the virtual IP address of the FHRP group.Answer
  2. B
    The standby router interface was assigned a higher numerical Spanning Tree Protocol bridge priority value, causing the switch port to remain in a blocking state.
  3. C
    The client workstations and the virtual default gateway address were assigned to different IP subnets during the DHCP lease initialization.
  4. D
    A native VLAN mismatch on the switch trunk port prevented Virtual Router Redundancy Protocol hello packets from reaching the standby router.

Answer

The DHCP scope is distributing the physical interface IP address of the primary router as the default gateway rather than the virtual IP address of the FHRP group.
The correct answer identifies that First Hop Redundancy Protocols (such as HSRP, VRRP, or CARP) rely on end hosts pointing to a shared Virtual IP (VIP) as their default gateway. If the DHCP scope is mistakenly configured to distribute the physical interface IP of the primary router, workstations will bypass the virtual gateway abstraction. Consequently, when the primary router shuts down, client traffic continues targeting the physical IP of the offline router, resulting in total loss of external connectivity despite the standby router successfully assuming the active state.

Step-by-Step Solution

1
Analyze the operational role of First Hop Redundancy Protocols (FHRPs) like HSRP or VRRP.
FHRP creates a shared Virtual IP (VIP) and Virtual MAC address shared between active and standby routers.
End devices must send traffic to the VIP so that whichever router is active receives and forwards the packets.
2
Evaluate host behavior during the failover event.
Host workstations continued sending frames to the physical IP/MAC of the powered-off primary router.
This indicates hosts were pointing directly to the primary router's physical IP address instead of the FHRP Virtual IP.
3
Identify the root configuration error in network infrastructure services.
The DHCP scope option 003 (Router / Default Gateway) was set to the physical interface IP instead of the VIP.
Correcting the DHCP scope to issue the VIP ensures host traffic seamlessly redirects to the standby router upon failover.

Key Concept

FHRP Virtual Gateway IP Configuration
Estimated Time:1m 30s
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Topics

High Availability and Redundancy Concepts

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