Introduction To System Debugging - 3Com MSR 50 Series Configuration Manual

2120 C 117: S
HAPTER YSTEM
Introduction to System
Debugging
M D
AINTAINING AND EBUGGING
The tracert command
By using the tracert command, you can trace the routers involved in delivering a
packet from source to destination. This is useful for identification of failed node(s)
in the event of network failure.
The tracert command involves the following steps in its execution:
1 The source device sends a packet with a TTL value of 1 to the destination device.
2 The first hop (the router 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 router.
3 The source device sends a packet with a TTL value of 2 to the destination device.
4 The second hop responds with a TTL-expired ICMP message, which gives the
source device the address of the second router.
5 The above process continues until the ultimate destination device is reached. In
this way, the source device can trace the addresses of all the routers involved to
get to the destination device.
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 screen.
As Figure 632 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.