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Difficulty: HardHigh Availability and Redundancy Concepts

A network administrator deploys two Layer 3 switches (Switch-Alpha and Switch-Beta) using Hot Standby Router Protocol (HSRP) to provide default gateway redundancy for a server subnet (10.50.10.0/2410.50.10.0/24). Switch-Alpha is designated as the primary gateway with an HSRP priority of 120, while Switch-Beta is configured with the default priority of 100. Interface tracking is enabled on Switch-Alpha to decrement its priority by 30 if its uplink interface fails. During a simulated link failure on Switch-Alpha's uplink, its priority drops to 90, allowing Switch-Beta (priority 100) to assume the active gateway role. However, after the uplink link on Switch-Alpha is fully restored and its priority returns to 120, Switch-Beta remains the active gateway while Switch-Alpha stays in standby mode. Which of the following root causes best explains why Switch-Alpha failed to reclaim the active gateway role upon uplink restoration?

  1. Switch-Alpha was not configured with the HSRP preempt command.Answer
  2. B
    The workstation hosts on the subnet were statically configured with Switch-Beta's physical IP address as their default gateway.
  3. C
    Switch-Beta was configured with the highest numerical Spanning Tree Protocol (STP) priority value.
  4. D
    The HSRP virtual IP address (VIP) was assigned directly to Switch-Alpha's physical interface.

Answer

Switch-Alpha was not configured with the HSRP preempt command.
In First Hop Redundancy Protocols such as HSRP, an active router that has assumed control will remain the active router even if another router on the segment comes online with a higher priority value. To force the higher-priority router (Switch-Alpha with priority 120) to reclaim the active role from Switch-Beta (priority 100) after link restoration, the `standby preempt` command must be explicitly configured on Switch-Alpha.

Step-by-Step Solution

1
Analyze HSRP priority behavior during uplink failure
Switch-Alpha's priority dropped from 120 to 90 (due to -30 decrement), which is lower than Switch-Beta's priority of 100. Switch-Beta legitimately took over the active role.
When a tracked object fails, the reduced priority allows a secondary router with higher current priority to take over active routing.
2
Analyze HSRP behavior upon link restoration
Switch-Alpha's priority recovered to 120, which is higher than Switch-Beta's priority of 100.
Restoring the tracked uplink interface cancels the priority decrement.
3
Identify the mechanism required for a higher-priority router to reclaim the active role
By default, HSRP does not allow a router with a higher priority to forcibly take over the active role from an currently active router unless preemption (`standby preempt`) is explicitly enabled on the higher-priority router.
Preemption allows a router with a higher priority to assume control as the active virtual router immediately upon becoming available.

Key Concept

FHRP Priority, Interface Tracking, and Preemption Mechanics
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Topics

High Availability and Redundancy Concepts

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