Cisco Systems 3750E Using Layer 2 Traceroute, Understanding Layer 2 Traceroute, Usage Guidelines

Chapter 45 Troubleshooting

Using Layer 2 Traceroute

Using Layer 2 Traceroute

These sections contain this information:

•Understanding Layer 2 Traceroute, page 45-17

•Usage Guidelines, page 45-17

•Displaying the Physical Path, page 45-18

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 45-17. 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 Chapter 27, “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.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		45-17