Assuming that preempt is not configured, when does a router in an HSRP group assume the role of the active router for the group?

A. A router in standby status will become the active router if it has a higher priority than the active router.
B. A router in standby status will become the active router when it does not detect three consecutive hello messages from the active router.

C. A router in standby status will become the active router when it does not detect any hello messages from the active router within the configured holdtime.

D. A router in listening status will become the active router when it does not detect any hello messages from the active router within the configured holdtime.

Explanation:

A router in standby status will become the active router when it does not detect any hello messages from the active router within the configured holdtime.

There are two ways for a router to become the HSRP active router. On startup, the router with the highest priority or IP address will become the active router. If the active router fails, the HSRP standby router is a candidate to become the next active HSRP router. Failure of the active router is detected by the loss of hello messages for a configurable amount of time referred to as holdtime. By default, hellos are sent every three seconds (hello time) and the holdtime is 10 seconds.

A router with the highest priority will be selected as the active router during the startup election process. If the active router fails and the standby router is promoted to be the active router, the first router will not immediately resume being the active router even if it has higher priority. This characteristic can be overridden with the configuration option of preempt. The router with the highest priority can initiate a coup to become the active router in the group if it has preempt enabled in the configuration. By default, all routers have an HSRP priority of 100. The range of values that can be assigned is 1 – 255.Other default values are:
● Standby holdtime is 10 seconds
● Standby track interface priority is 10

To illustrate these concepts, consider the following example. Router A is configured with a priority of 150 and Router B is configured with a priority of 100. Neither router is configured to preempt. If both routers were shut down and Router 5 was rebooted first, then Router B would become the active router. If Router A was then rebooted, it would not become the active router even though it has a higher priority than Router B, because it was NOT configured with the preempt command to allow it to assume the active role with a higher priority.

The six HSRP states are defined as follows:
● Initial state: All routers start in this state.
● Learn state: The router is in the learn state when it has not communicated with the active router. It does not know which router is the active router and does not know the IP address of the virtual router (if no HSRP IP address configured in the router).
● Listen state: Once the router hears from the active router and knows the virtual IP address, it enters the listen state. It is not the active or standby router.
● Speak state: After a router learns the IP address of the virtual router, it enters the speak state. It participates in the active and standby router election. It sends hello messages to the active router.
● Standby state: When the active router has been elected, the second router enters the standby state. This is the standby router and it will become the active router if the active router fails.
● Active state: The router is in active state when it is forwarding packets. It receives packets via the virtual IP address.

Objective:
Infrastructure Services
Sub-Objective:
Configure and verify first-hop redundancy protocols

References:
Internetworking Case Studies > Using HSRP for Fault-Tolerant IP Routing
Cisco > Cisco IOS IP Application Services Configuration Guide, Release 12.4 > Part 1: First Hop Redundancy Protocols > Configuring HSRP