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Difficulty: MediumFirst Hop Redundancy Protocols (HSRP and VRRP)

A network administrator configures HSRP group 10 on two Cisco IOS routers to provide high availability for the 192.168.10.0/24192.168.10.0/24 network.

Router R1 configuration:
text interface GigabitEthernet0/1 ip address 192.168.10.2 255.255.255.0 standby 10 ip 192.168.10.1 standby 10 priority 115 standby 10 preempt standby 10 track GigabitEthernet0/0 30

Router R2 configuration:
text interface GigabitEthernet0/1 ip address 192.168.10.3 255.255.255.0 standby 10 ip 192.168.10.1 standby 10 priority 100 standby 10 preempt

Initially, both routers and interfaces are fully operational, and R1 is the Active router. If interface GigabitEthernet0/0 on R1 experiences a link failure, which router will become the Active router for HSRP group 10, and what is R1's effective priority?

  1. R2 becomes the Active router because R1's priority decrements to 85, which is lower than R2's priority of 100.Answer
  2. B
    R1 remains the Active router because R1's priority decrements to 100, resulting in a tie where the current Active router retains its role.
  3. C
    R1 remains the Active router because R2 lacks interface tracking and cannot preempt an active router during an uplink failure.
  4. D
    R2 becomes the Active router because HSRP tracking immediately disables the standby group on R1 upon uplink failure.

Answer

R2 becomes the Active router because R1's priority decrements to 85, which is lower than R2's priority of 100.
When the tracked uplink GigabitEthernet0/0 on R1 fails, HSRP interface tracking decrements R1's priority from 115 by 30, bringing its effective priority down to 85. Since R2 has preemption enabled and its priority of 100 is higher than R1's new priority of 85, R2 preempts R1 and becomes the Active router for HSRP group 10.

Step-by-Step Solution

1
Calculate the initial priorities and state of both HSRP routers.
R1 initial priority is 115 (Active). R2 priority is 100 (Standby). Both have preemption enabled.
Higher priority wins the Active role when preemption is enabled.
2
Evaluate the impact of GigabitEthernet0/0 failure on R1.
R1's tracked interface drops, reducing R1's priority by 30: 115−30=85115 - 30 = 85.
The command 'standby 10 track GigabitEthernet0/0 30' decrements the priority by 30 upon link down.
3
Determine the outcome of HSRP preemption.
R2's priority (100) is now higher than R1's effective priority (85). R2 sends coup messages and becomes Active.
R2 has 'standby 10 preempt' configured, allowing it to assume the Active role whenever its priority exceeds the current Active router's priority.

Key Concept

HSRP Priority Tracking and Preemption
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Topics

First Hop Redundancy Protocols (HSRP and VRRP)

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