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Difficulty: MediumFirst Hop Redundancy Protocols (HSRP and VRRP)

A network engineer enables preemption on router R1 (configured with HSRP priority 120120) while router R2 (configured with HSRP priority 100100) is currently acting as the Active gateway for HSRP Group 1. In what chronological sequence do the protocol events occur as router R1 takes over the Active role?

  1. 1Router R1 receives HSRP Hello packets from active router R2 and identifies R2's operational priority of 100100.
  2. 2Router R1 compares its local priority (120120) to the active router's priority (100100) and invokes preemption logic.
  3. 3Router R1 transmits an HSRP Coup message to notify router R2 that a higher-priority router is asserting control.
  4. 4Router R2 receives the takeover notification, relinquishes the Active role, and broadcasts an HSRP Resign message.
  5. 5Router R1 transitions to the Active state and sends a Gratuitous ARP (GARP) to update switch CAM tables with the virtual MAC address.

Answer

The correct sequence for HSRP active gateway preemption is: 1) Router R1 receives HSRP Hello packets from R2, 2) Router R1 compares priorities and triggers preemption, 3) Router R1 sends an HSRP Coup message, 4) Router R2 relinquishes the role and sends an HSRP Resign message, and 5) Router R1 becomes Active and broadcasts a Gratuitous ARP.
When a router configured with a higher HSRP priority and preemption comes online, it must first receive Hello packets from the current Active router to detect the priority difference. Once detected, it sends an HSRP Coup packet. The lower-priority router acknowledges this by issuing an HSRP Resign packet and moving to Standby. Finally, the new router enters the Active state and issues a Gratuitous ARP (GARP) to update Layer 2 switch forwarding tables.

Step-by-Step Solution

1
Identify active discovery
R1 processes Hello packets from current active router R2 containing priority 100100.
Before preempting, the candidate router must learn the current active router's operating parameters.
2
Evaluate preemption condition
R1 determines its priority (120120) is higher than R2's (100100) and preemption is configured.
Preemption allows a router with a higher priority to take over the Active state automatically.
3
Send takeover notification
R1 generates and transmits an HSRP Coup message.
The Coup message signals to the existing Active router that a superior router is taking over.
4
Relinquish active role
R2 processes the Coup message, transitions to Standby, and emits a Resign message.
The lower-priority active router steps down upon verifying the higher priority in the Coup message.
5
Finalize activation and update network paths
R1 enters Active state and sends a Gratuitous ARP (GARP).
The GARP forces adjacent switches to map the virtual MAC address to R1's port, preventing blackholing of user traffic.

Key Concept

HSRP Preemption Mechanics and Message Sequence
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Topics

First Hop Redundancy Protocols (HSRP and VRRP)

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