Table of Contents
Chapter 20
Checking Status and Connectivity
Using Layer 2 Traceroute
Layer 2 traceroute allows you to identify the physical path that a packet takes from a source to a
destination. Layer 2 traceroute determines the path by looking at the forwarding engine tables of the
switches in the path.
Information is displayed about all Catalyst 4500 series, Catalyst 5000 family, and Catalyst 6500 series
switches that are in the path from the source to the destination.
Layer 2 Traceroute Usage Guidelines
This section lists the guidelines for the Layer 2 traceroute:
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Identifying a Layer 2 Path
To identify a Layer 2 path, perform one of these tasks in privileged mode:
Task
Trace a Layer 2 path using MAC
addresses.
Trace a Layer 2 path using IP addresses or
IP aliases.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
78-15908-01
Layer 2 traceroute works for unicast traffic only.
You must enable Cisco Discovery Protocol (CDP) on all of the Catalyst 4500 series, Catalyst 5000
family, and Catalyst 6500 series switches in the network. (See
information about enabling CDP.) If any devices in the path are transparent to CDP, l2trace will not be
able to trace the Layer 2 path through those devices.
You can use this Layer 2 traceroute from a switch that is not in the Layer 2 path between the source
and the destination; however, all of the switches in the path, including the source and destination,
must be reachable from the switch.
All switches in the path must be reachable from each other.
You can trace a Layer 2 path by specifying the source and destination IP addresses (or IP aliases) or
the MAC addresses. If the source and destination belong to multiple VLANs and you specify MAC
addresses, you can also specify a VLAN.
The source and destination switches must belong to the same VLAN.
The maximum number of hops that an l2trace query will try is 10; this includes hops involved in
source tracing.
Layer 2 traceroute does not work with Token Ring VLANs, when multiple devices are attached to
one port through hubs, or when multiple neighbors are on a port.
Chapter 21, "Configuring CDP,"
Command
l2trace {src-mac-addr} {dest-mac-addr} [vlan] [detail]
l2trace {src-ip-addr} {dest-ip-addr} [detail]
Using Layer 2 Traceroute
for
20-11
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