Foundry NetIron M2404C and M2404F Metro Access Switches
Troubleshooting and Monitoring (Rev. 03)
Diagnosing Connectivity Problems
© 2008 Foundry Networks, Inc.
Page 9 of 55
The user 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,
through which the traffic passes on its way to the destination.
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-to-live-exceeded” message to the sender. Traceroute determines the address of the first hop
by examining the source address field of the ICMP “time-to-live-exceeded” message.
To identify the next hop, traceroute sends a UDP packet with a TTL value of 2. The first router
decreases 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-to-live-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). By default the TTL
value is 64.
To determine when a datagram reaches its destination, traceroute sets the UDP destination port
number in the datagram to a very large value that the destination host is unlikely to be using. When
a host receives a self-destined datagram containing a destination port number that is unused
locally, it sends an ICMP “port unreachable” error to the source. Because all errors except “port
unreachable” errors come from intermediate hops, the receipt of a “port unreachable” error means
this message was sent by the destination.
Command Syntax
device-name
#
traceroute
{
A.B.C.D
|
HOST-NAME
}
[
ttl
<
ttl
>] [
timeout
<
timeout
>]
Argument Description
A.B.C.D
Destination IP address.
HOST-NAME
Destination host name (using DNS server).
ttl
<
ttl
>
(Optional). Defines the numbers of routers that allow the
traceroute
command to pass when it looks for the specified IP address in range
<1-255>. The default TTL is 64.
timeout
<
timeout
>
(Optional). Defines the length of time (in seconds) during which an
answer to a
traceroute
request can be received in range <1-600>
seconds. The default timeout is 2 seconds.
Example
device-name
#
traceroute 192.118.82.140
1 : 10ms. 20ms. 10ms. - Hop [212.29.220.193]
2 : 50ms. 40ms. 40ms. - Hop [10.96.96.1]
3 : 60ms. 95ms. 95ms. - Hop [212.29.196.109]
4 : 60ms. 60ms. 100ms. - Hop [206.49.94.116]
5 : 225ms. 100ms. 220ms. - Hop [212.29.206.214]
6 : 60ms. 60ms. 55ms. - Hop [212.29.206.66]
7 : 60ms. 60ms. 60ms. - Hop [212.29.206.210]
8 : 60ms. 60ms. 65ms. - Hop [212.150.63.186]
9 : 80ms. 85ms. 80ms. - Hop [192.118.68.17]
10 : 65ms. 70ms. 70ms. - Target [192.118.82.140]