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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter32 Troubleshooting Diagnosing Connectivity Problems
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 24, “Configuring CDP.”
•A switch is reachable from another switch when you can test connect ivity by using the pi ng
privileged EXEC command. All switches in the physical path must be reac habl e f rom ea ch oth er.
•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 th e
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 a nd 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 whe n th e specified so urc e a nd
destination IP addresses belong to the same subnet. When you specify the IP a ddresse s, th e s witc h
uses Address Resolution Protocol (ARP) to associate the IP addresses with the corresponding MAC
addresses and the VLAN IDs.
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If an ARP entry exists for the specified IP address, the switch uses the associated MA C ad dress
and identifies the physical path.
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If an ARP entry does not exist, the switch sends an ARP query and tries to resolve the IP
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 neigh bors
are detected on a port), the Layer 2 traceroute feature is not supp ort ed. Wh en m ore than on e CDP
neighbor is detected on a port, the Layer 2 path is not identified, and an error message appear s.
•This feature is not supported in Token Ring VLANs.