• You can enter the traceroute mac or the traceroute mac ip privileged EXEC command on a device that is not in the physical path from the source device to the destination device. All devices 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 device 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 device uses the associated MAC address and identifies the physical path.
• If an ARP entry does not exist, the device 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.
• Layer 2 traceroute opens a listening socket on the User Datagram Protocol (UDP) port 2228 that can be accessed remotely with any IPv4 address, and does not require any authentication. This UDP socket allows to read VLAN information, links, presence of particular MAC addresses, and CDP neighbor information, from the device. This information can be used to eventually build a complete picture of the Layer 2 network topology. • Layer 2 traceroute is disabled by default and can be enabled by running the l2 traceroute command in global configuration mode.
To disable Layer 2 traceroute, use the no l2 traceroute command in global configuration mode 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 Device 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 Device is the destination of the traceroute, it is displayed as the final destination in the traceroute output.
Intermediate devices 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 Device is a multilayer Device that is routing a particular packet, this device shows up as a hop in the traceroute output. Troubleshooting the Software Configuration IP Traceroute Troubleshooting the Software Configuration 4