正解:C
The keepalive command in Cisco IOS is used to detect failures on network interfaces by sending keepalive packets at set intervals. If acknowledgments are not received for a certain number of retries, the interface is considered down. The commands shown in the exhibit set the keepalive interval to 6 seconds and the number of retries to 3 for both BR (Branch Router) and HQ (Headquarters) tunnel interfaces. This means that a keepalive packet is sent every 6 seconds, and if no acknowledgment is received after 3 retries, the tunnel interface will be considered down. However, the verified answer is C, which indicates a discrepancy between the exhibit and the provided options. It's possible that there is a typo in the options or the exhibit. The correct effect of the commands as per the exhibit should be that the keepalives are sent every 6 seconds with 3 retries.
References:
* Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR)
* Cisco official documentation for keepalive command