Cisco Systems 3750E manual Using IP Traceroute, Displaying the Physical Path, 45-18

Chapter 45 Troubleshooting

Using IP Traceroute

•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 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.

•This feature is not supported in Token Ring VLANs.

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:

•tracetroute mac [interface interface-id]{source-mac-address} [interface interface-id]{destination-mac-address} [vlan vlan-id] [detail]

•tracetroute mac ip {source-ip-address source-hostname}{destination-ip-address destination-hostname} [detail]

For more information, see the command reference for this release.

Using IP Traceroute

These sections contain this information:

•Understanding IP Traceroute, page 45-18

•Executing IP Traceroute, page 45-19

Understanding IP Traceroute

You can use IP traceroute to identify the path that packets take through the network on a hop-by-hop basis. The command output displays all network layer (Layer 3) devices, such as routers, that the traffic passes through on the way to the destination.

Your switches can participate as the source or destination of the traceroute privileged EXEC command and might or might not appear as a hop in the traceroute command output. If the switch is the destination of the traceroute, it is displayed as the final destination in the traceroute output. Intermediate switches do not show up in the traceroute output if they are only bridging the packet from one port to another within the same VLAN. However, if the intermediate switch is a multilayer switch that is routing a particular packet, this switch shows up as a hop in the traceroute output.

The traceroute privileged EXEC command uses the Time To Live (TTL) field in the IP header to cause routers and servers to generate specific return messages. Traceroute starts by sending a User Datagram Protocol (UDP) datagram to the destination host with the TTL field set to 1. If a router finds a TTL value

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
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