The first hop (the Layer 3 device that first receives the packet) responds by sending a TTL-expired
ICMP message to the source, with its IP address encapsulated. In this way, the source device can
get the address of the first Layer 3 device.
The source device sends a packet with a TTL value of 2 to the destination device.
The second hop responds with a TTL-expired ICMP message, which gives the source device the
address of the second Layer 3 device.
The above process continues until the ultimate destination device is reached. In this way, the
source device can trace the addresses of all the Layer 3 devices involved to get to the destination
Introduction to System Debugging
The device provides various debugging functions. For the majority of protocols and features supported,
the system provides corresponding debugging information to help users diagnose errors.
The following two switches control the display of debugging information:
Protocol debugging switch, which controls protocol-specific debugging information.
Screen output switch, which controls whether to display the debugging information on a certain
illustrates, suppose the device can provide debugging for the three modules 1, 2, and 3.
Only when both the protocol debugging switch and the screen output switch are turned on can
debugging information be output on a terminal.
Figure 1-1 The relationship between the protocol and screen debugging switch
Outputting debugging information to a terminal is most commonly used. You can also configure to
output debugging information to other directions. For detailed configuration, refer to Information Center
Configuration in the System Volume.