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Chapter 19
Checking Port Status and Connectivity
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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. l2trace {src-mac-addr} {dest-mac-addr} [vlan] [detail]
Trace a Layer 2 path using IP addresses or
IP aliases.
This example shows the source and destination MAC addresses specified, with no VLAN specified but
with the detail option specified. For each Catalyst 4000, 5000, and 6000 family switch found in the path,
the output shows the device type, device name, device IP address, in port name, in port speed, in port
duplex mode, out port name, out port speed, and out port duplex mode.
Console> (enable) l2trace 00-01-22-33-44-55 10-22-33-44-55-66 detail
l2trace vlan number is 10.
00-01-22-33-44-55 found in C4000 named wiring-1 on port 4/1 10Mb half duplex
C4000:wiring-1:192.168.242.10:4/1 10Mb half duplex -> 5/2 100MB full duplex
C4000:backup-wiring-1:192.168.242.20:1/1 100Mb full duplex -> 3/1 100MB full duplex
C5000:backup-core-1:192.168.242.30:4/1 100 MB full duplex -> 1/1 100MB full duplex
C6000:core-1:192.168.242.40:1/1 100MB full duplex -> 2/1 10MB half duplex.
10-22-33-44-55-66 found in C6000 named core-1 on port 2/1 10MB half duplex.
Using IP Traceroute
These sections describe how to use IP traceroute:
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Understanding How IP Traceroute Works
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.
Switches can participate as the source or destination of the traceroute command but will not appear as
a hop in the traceroute command output.
The traceroute 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 of 1 or 0, it
78-12647-02
The maximum number of hops an l2trace query will try is 10; this includes hops involved in source
tracing.
The Layer 2 Traceroute utility does not work with Token Ring VLANs, or when multiple devices are
attached to one port via hubs, or when multiple neighbors are on a port.
Understanding How IP Traceroute Works, page 19-11
Executing IP Traceroute, page 19-12
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
Command
l2trace {src-ip-addr} {dest-ip-addr} [detail]
Using IP Traceroute
19-11
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Troubleshooting
