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Difficulty: HardFirst Hop Redundancy Protocols (HSRP and VRRP)

A network administrator has configured HSRPv2 between two routers, R1 and R2. Router R1 is currently the Active router with a configured priority of 120, preemption enabled, and tracking on its primary WAN interface configured to decrement priority by 30 upon link failure. Router R2 is currently the Standby router with a configured priority of 105 and preemption enabled. When R1's tracked WAN interface fails, a precise sequence of operational events occurs across the routers and local LAN switches to complete the failover. What is the correct chronological sequence of these events from first to last?

  1. 1Router R1's tracked WAN interface goes down, triggering an internal decrement of R1's operational priority from 120 to 90.
  2. 2Router R1 transmits an HSRP Hello packet containing its newly decremented priority value (90) across the local LAN.
  3. 3Router R2 receives R1's Hello packet, identifies that R1's priority (90) is lower than its own priority (105), and issues an HSRP Coup message.
  4. 4Router R2 transitions from Standby to Active state and broadcasts a Gratuitous ARP (GARP) frame containing the virtual IP and virtual MAC address.
  5. 5Router R1 receives R2's Active state messages with priority 105 and demotes its state from Active to Standby.

Answer

The correct sequence of events is: (1) R1's tracked WAN interface fails and internal priority decrements to 90, (2) R1 advertises priority 90 in an HSRP Hello packet, (3) R2 observes R1's lower priority and transmits an HSRP Coup message due to preemption, (4) R2 assumes Active status and transmits a Gratuitous ARP (GARP) frame to update switch CAM tables, and (5) R1 processes R2's higher-priority election and transitions to Standby status.
The failover process begins when R1's tracked interface fails, immediately decreasing its local priority from 120 to 90. R1 advertises this new priority of 90 in its subsequent HSRP Hello multicast packet. R2 reads R1's Hello packet, notices its own priority of 105 is higher than R1's 90, and generates an HSRP Coup message because preemption is enabled. R2 then transitions to Active status and broadcasts a Gratuitous ARP (GARP) frame so adjacent switches update their MAC address tables for the virtual MAC address. Finally, R1 receives R2's Active message and transitions to Standby status.

Step-by-Step Solution

1
Identify the initial interface failure and local priority recalculation.
R1's priority is decremented by 30 (120 - 30 = 90).
HSRP interface tracking continuously monitors the specified interface status and modifies the router priority locally upon state change.
2
Determine how priority changes are communicated across the subnet.
R1 sends an HSRP Hello packet advertising priority 90.
HSRP state updates and priority shifts must be transmitted via multicast Hello messages to peer routers.
3
Evaluate peer router reaction and preemption mechanism.
R2 compares priorities (105 vs 90) and sends a Coup message.
Because preemption is enabled on R2, R2 immediately attempts to claim the Active role when it sees an Active router with lower priority.
4
Analyze Layer 2 forwarding convergence.
R2 shifts to Active state and sends a Gratuitous ARP (GARP).
Layer 2 switches must learn that the virtual MAC address is now reachable via R2's switch port, which is accomplished via GARP frame broadcast.
5
Verify final state transition of the original Active router.
R1 transitions to Standby state.
R1 recognizes R2 as a valid Active router with higher priority (105 > 90) and completes demotion.

Key Concept

HSRP Interface Tracking, Preemption Coup Mechanics, and GARP Convergence
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Topics

First Hop Redundancy Protocols (HSRP and VRRP)

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