49-16
Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter49 Troubleshooting
Using Layer 2 Traceroute
To end a ping session, enter the escape sequence (Ctrl-^ X by default). Simultaneously press and release
the Ctrl, Shift, and 6 keys and then press the X key.
Using Layer 2 Traceroute
These sections contain this information:
•Understanding Layer 2 Traceroute, page49-16
•Usage Guidelines, page 49-16
•Displaying the Physical Path, page49-17

Understanding Layer 2 Traceroute

The Layer 2 traceroute feature allows the switch to identify the physical path that a packet takes from a
source device to a destination device. Layer 2 traceroute supports only unicast source and destination
MAC addresses. It finds the path by using the MAC address tables of the switches in the path. When the
switch detects a device in the path that does not support Layer 2 traceroute, the switch continues to send
Layer 2 trace queries and lets them time out.
The switch can only identify the path from the source device to the destination device. It cannot identify
the path that a packet takes from source host to the source device or from the destination device to the
destination host.

Usage Guidelines

These are the Layer 2 traceroute usage guidelines:
•Cisco Discovery Protocol (CDP) must be enabled on all the devices in the network. For Layer 2
traceroute to function properly, do not disable CDP.
For a list of switches that support Layer 2 traceroute, see the “Usage Guidelines” section on
page 49-16. If any devices in the physical path are transparent to CDP, the switch cannot identify
the path through these devices. For more information about enabling CDP, see Chapter26,
“Configuring CDP.”
•A switch is reachable from another switch when you can test connectivity by using the ping
privileged EXEC command. All switches in the physical path must be reachable from each other.
•The maximum number of hops identified in the path is ten.
•You can enter the traceroute mac or the traceroute mac ip privileged EXEC command on a switch
that is not in the physical path from the source device to the destination device. All switches in the
path must be reachable from this switch.
•The traceroute mac command output shows the Layer 2 path only when the specified source and
destination MAC addresses belong to the same VLAN. If you specify source and destination MAC
addresses that belong to different VLANs, the Layer 2 path is not identified, and an error message
appears.
•If you specify a multicast source or destination MAC address, the path is not identified, and an error
message appears.