1. If the router does not receive a response from the target host, it will send ARP requests for the next four packets. If the target host’s MAC address has not been resolved by the time the sixth packet arrives, the router, by default, will discard the sixth and subsequent packets over the next 20 seconds and return a host unreachable ICMP message to the source host.
2. The router sends out an ARP query to the network segment, but no computer responds. The router then determines that the target host does not exist in that segment, so it returns an ICMP message to the source computer stating that the target host is unreachable, notifying the source host of the issue, and simultaneously discards the original data packet. The problem is now clear: the ICMP messages logged by the router are all messages sent back to the source address. Analysis of the captured data reveals that these external hosts are primarily looking for three specific internal computers.
3. The target host’s ports are fixed between 6881 and 6889, which happen to be commonly used ports for the now-popular BT downloading. When these hosts use BT to download, they leave records on the BT server so that other hosts can download resources from them. When these hosts are shut down, the router informs others that it cannot find them.
4. Since the log service records information at Layer 3 and above, and the packets received by the router are dropped at Layer 2, these abnormal incoming packets are not recorded in the logs. To reduce the router’s log volume, use the ip disable icmp-messages destination-unreachables command in configuration mode to prevent the forwarding of such messages. Both faults were triggered by ICMP, and from a certain perspective, neither was a system configuration issue but rather caused by external factors. Troubleshooting such faults requires a certain amount of analysis to identify the cause, after which corresponding configurations can be made to resolve them.
5. First, use the router’s dedicated CONSOLE cable to connect the laptop’s COM port to the router’s CONSOLE port, then launch a terminal emulator on the laptop to establish