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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
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Chapter 20 Checking Status and Connectivity
Using IP Traceroute
This example shows the source and destination MAC addresses specified, with no VLAN specified but
with the detail keyword specified. For each Catalyst 4500 series, 5000 family, and 6500 series 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 C4000 named core-1 on port 2/1 10MB half duplex.
Console> (enable)
Using IP Traceroute
The next two sections describe how to use IP traceroute.
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
drops the datagram and sends back an Internet Control Message Protocol (ICMP) time-exceeded
message to the sender. IP traceroute determines the address of the first hop by examining the source
address field of the ICMP time-exceeded message.
To identify the next hop, traceroute sends a UDP packet with a TTL value of 2. The first router
decrements the TTL field by 1 and sends the datagram to the next router. The second router sees a TTL
value of 1, discards the datagram, and returns the time-exceeded message to the source. This process
continues until the TTL is incremented to a value large enough for the datagram to reach the destination
host (or until the maximum TTL is reached).
To determine when a datagram reaches its destination, traceroute sets the UDP destination port in the
datagram to a very large value that the destination host is unlikely to be using. When a host receives a
datagram with an unrecognized port number, it sends an ICMP port unreachable error to the source. This
message indicates to IP traceroute that it has reached the destination.