Layer 2 Traceroute Usage Guidelines - Cisco Catalyst 4500 Series Software Configuration Manual

Using Layer 2 Traceroute
If you want the switch to trace the path from a host on a source device to a host on a destination device,
the switch can identify only 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.
These sections describe how to use the Layer 2 traceroute feature:
•
•

Layer 2 Traceroute Usage Guidelines

These are the Layer 2 traceroute usage guidelines:
•
•
•
•
•
•
•
•
•
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
10-10
Layer 2 Traceroute Usage Guidelines, page 10-10
Running Layer 2 Traceroute, page 10-11
CDP must be enabled on all the devices in the network. For Layer 2 traceroute to function 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.
For more information about enabling CDP, see
Note
All switches in the physical path must have IP connectivity. When a switch is reachable from another
switch, you can test connectivity by using the ping command in privileged EXEC mode.
The maximum number of hops identified in the path is ten.
You can enter the traceroute mac or the traceroute mac ip command in privileged EXEC mode 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 address with the corresponding MAC
address and the VLAN ID.
– If an ARP entry exists for the specified IP address, the switch uses the associated MAC address
and identifies the physical path.
– 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 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.
Chapter 10 Checking Port Status and Connectivity
Chapter 31, "Configuring CDP."