Usage Guidelines; Displaying The Physical Path - Cisco Catalyst 2928 Software Configuration Manual

Chapter 33 Troubleshooting

Usage Guidelines

These are the Layer 2 traceroute usage guidelines:
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Displaying the Physical Path

You can display physical path that a packet takes from a source device to a destination device by using
one of these privileged EXEC commands:
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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
page 33-15. 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
"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 the Address Resolution Protocol (ARP) to associate the IP addresses with the corresponding
MAC addresses and the VLAN IDs.
If an ARP entry exists for the specified IP address, the switch uses the associated MAC address
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and identifies the physical path.
If an ARP entry does not exist, the switch sends an ARP query and tries to resolve the IP
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address. If the IP address is not resolved, the path is not identified, and an error message
appears.
When multiple devices are attached to one port through hubs (for example, multiple CDP neighbors
are detected on a port), the Layer 2 traceroute feature is not supported. When more than one CDP
neighbor is detected on a port, the Layer 2 path is not identified, and an error message appears.
This feature is not supported in Token Ring VLANs.
tracetroute mac [interface interface-id] {source-mac-address} [interface interface-id]
{destination-mac-address} [vlan vlan-id] [detail]
"Usage Guidelines" section on
Catalyst 2928 Switch Software Configuration Guide
Using Layer 2 Traceroute
Chapter 23,
33-15