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134 CHAPTER 6: CONFIGURING AND MANAGING IP INTERFACES AND SERVICES
To identify the next hop, traceroute again sends a UDP packet, but this
time 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 has reached its destination, traceroute
sets the UDP destination port in the datagram to a very large value, one
that the destination host is unlikely to be using. In addition, 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 the
traceroute facility that it has reached the destination.
To trace a route to a destination subnet, use the following command:
traceroute host [dnf] [no-dns] [port port-num] [queries num]
[size size] [ttl hops] [wait ms]
To trace the route to host server1, type the following command:
WX1200# traceroute server1
traceroute to server1.example.com (192.168.22.7), 30 hops max, 38 byte packets
1 engineering-1.example.com (192.168.192.206) 2 ms 1 ms 1 ms
2 engineering-2.example.com (192.168.196.204) 2 ms 3 ms 2 ms
3 gateway_a.example.com (192.168.1.201) 6 ms 3 ms 3 ms
4 server1.example.com (192.168.22.7) 3 ms * 2 ms
In this example, server1 is four hops away. The hops are listed in order,
beginning with the hop that is closest to the WX switch and ending with
the route’s destination. (For information about the command options, see
the Wireless LAN Switch and Controller Command Reference.)