Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 229)

Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide 205
53-1003031-02
IP Traceroute over MPLS
1
Standard traceroute
Traceroute is a diagnostic utility that allows the user to troubleshoot a network path by iteratively
sending Internet Control Message Protocol (ICMP) packets through an IP network from a source to
a destination. Packets have a defined TTL and sent to a port that is known to be invalid on the
destination device (usually above 3300). At each hop, the transit device decrements the
Time-to-Live (TTL) value contained in the IP header portion of the datagram by one. Based on the
remaining TTL value, the device performs one of the following actions:
• TTL > 0: The device passes the packet to the next device using standard IP routing protocols.
• TTL = 0: The device drops the packet, which is now expired, and returns an ICMP response of
type 11, ttl-exceeded, along with a portion of the original datagram to the source device that
originated the traceroute probe.
With each traceroute iteration, the source device increments the TTL value of the packets by one.
This causes the packets to be forwarded another hop to the next transit device until the TTL is large
enough for the probe to reach the destination.
When the destination device receives the packets, it attempts to connect to the port specified in
the ICMP. The attempt fails because the port is invalid. The recipient generates an ICMP message
of type 3, destination unreachable, and sends it back to the source device that originated the
probe.
Based on the ICMP messages received during the traceroute operation, the source device obtains
the following information:
• The number of hops traversed by the traceroute probe until it reaches its destination.
• The IP addresses for each of the devices the probes encounter along their path from the
source to the destination.
• The round-trip time (RTT) for each successive probe. The RTT value is the sum of the time a
packet travels until it expires, plus the time the ICMP message takes to return to the source.
The following example illustrates the output of the traceroute command traversing seven hops in a
standard IP network.
Brocade# traceroute 10.125.31.70
Type Control-c to abort
Tracing the route to IP node (10.157.22.199) from 1 to 30 hops
1 4 ms <1 ms <1 ms 10.31.20.25
2 <1 ms <1 ms <1 ms 10.16.200.121
3 <1 ms <1 ms <1 ms 10.110.111.102
4 <1 ms <1 ms <1 ms 10.49.131.1
5 <1 ms <1 ms <1 ms 10.49.130.18
6 <1 ms <1 ms 1 ms 10.125.199.61
7 1 ms 3 ms 2 ms 10.125.31.70T
Each line of output represents a hop along the IP network path. For each packet sent, the
traceroute command records the RTT in milliseconds and the IP address of the device that
returned the ICMP ttl-exceeded message. An asterisk (*) indicates that no information could be
obtained for the specified hop or the traceroute command timed out.
For traceroute command syntax information, refer to the Brocade MLX Series and Brocade NetIron
Diagnostic Guide.