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Difficulty: HardHigh Availability and Redundancy Concepts

An enterprise data center utilizes two edge routers, Edge-1 and Edge-2, configured with Hot Standby Router Protocol (HSRP) to provide default gateway redundancy for internal LAN clients. Edge-1 is configured with an HSRP priority of 120, while Edge-2 is configured with the default priority of 100. Both routers connect to downstream switches via their LAN interfaces and to separate ISP providers via their WAN interfaces. During a line cut on Edge-1's WAN circuit, internal LAN clients report a complete loss of internet connectivity. Network monitoring shows that Edge-1 remains the active HSRP router and continues accepting LAN traffic. Which of the following is the most likely cause of this failover failure?

  1. HSRP interface tracking was not configured on Edge-1's WAN interface to decrement its priority upon link failure.Answer
  2. B
    The downstream LAN hosts were configured with Edge-1's physical IP address instead of the HSRP virtual IP address.
  3. C
    Spanning Tree Protocol failed to elect Edge-2 as the root bridge for the WAN VLAN following the ISP circuit cut.
  4. D
    Edge-2 was configured with a default gateway address residing on a different subnet than the downstream LAN hosts.

Answer

HSRP interface tracking was not configured on Edge-1's WAN interface to decrement its priority upon link failure.
The correct answer highlights the necessity of FHRP interface or object tracking. By default, HSRP only monitors the local interface where standby groups are configured. If an upstream WAN interface fails while the local LAN interface stays active, HSRP priority remains unchanged unless interface tracking is configured to decrement the priority below the standby router's priority.

Step-by-Step Solution

1
Analyze the HSRP state and network symptoms.
Edge-1 has a higher priority (120) than Edge-2 (100) and remains the active HSRP router despite losing its WAN interface.
By default, HSRP routers only monitor the operational state of the interface on which HSRP is running (the LAN interface).
2
Evaluate why Edge-1 did not yield the active role to Edge-2.
Because the LAN interface on Edge-1 remained operational, Edge-1 continued transmitting HSRP hellos with priority 120.
Without tracking the upstream WAN interface status, HSRP has no mechanism to know that outbound internet traffic through Edge-1 will be black-holed.
3
Identify the required feature to resolve upstream failover issues.
Configuring HSRP interface/object tracking on Edge-1 decrements its priority (e.g., by 30 points to 90) when the WAN link drops.
When Edge-1's priority drops below Edge-2's priority (100), preemption allows Edge-2 to seamlessly assume the active role.

Key Concept

FHRP Interface/Object Tracking
Estimated Time:2m 0s
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Topics

High Availability and Redundancy Concepts

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