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Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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Chapter 31 Troubleshooting
Diagnosing Connectivity Problems
Using Layer 2 Traceroute
This section describes this information:
• Understanding Layer 2 Traceroute, page 31-16
• Usage Guidelines, page 31-16
• Displaying the Physical Path, page 31-17

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 determines 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 functional properly, do not disable CDP. If any devices in the physical path are
transparent to CDP, the switch cannot identify the path through these devices.
Note For more information about enabling CDP, see Chapter 23, “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.
• If the source or destination MAC address belongs to multiple VLANs, you must specify the VLAN
to which both the source and destination MAC addresses belong. If the VLAN is not specified, the
path is not identified, and an error message appears.
• The traceroute mac ip command output shows the Layer 2 path when the specified source and
destination IP addresses belong to the same subnet. When you specify the IP addresses, the switch
uses Address Resolution Protocol (ARP) to associate the IP addresses with the corresponding MAC
addresses and the VLAN IDs.