Usage Guidelines - Cisco 7604 Configuration Manual

Usage Guidelines

Usage Guidelines
When using the Layer 2 traceroute utility, follow these guidelines:
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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
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Cisco Discovery Protocol (CDP) must be enabled on all the devices in the network. For the Layer 2
traceroute utility to function properly, do not disable CDP. If any devices in the Layer 2 path are
transparent to CDP, the Layer 2 traceroute utility cannot identify these devices on the path.
For more information about CDP, see
Note
A router is defined as reachable from another router when you can test connectivity by using the
ping privileged EXEC command. All devices in the Layer 2 path must be mutually reachable.
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 router
that is not in the Layer 2 path from the source device to the destination device. All devices in the
path must be reachable from this router.
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 Layer 2
traceroute utility 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 Layer 2 traceroute utility uses the
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associated MAC address and identifies the Layer 2 path.
If an ARP entry does not exist, the Layer 2 traceroute utility sends an ARP query and tries to
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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 utility terminates at that hop and displays an error
message.
The Layer 2 traceroute utility is not supported in Token Ring VLANs.
Chapter 57 Using the Layer 2 Traceroute Utility
Chapter 48, "Configuring CDP."
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