3Com CoreBuilder 3500, Implementation Manual, Page: 532 / 592

532 C
HAPTER
18: D
EVICE
M
ONITORING
traceRoute
Use the traceRoute feature to track the route of an IP packet through the
network. TraceRoute information includes all of the nodes in the network
through which a packet passes to get from its origin to its destination.
The traceRoute feature uses the IP time-to-live (TTL) field in User
Datagram Protocol (UDP) probe packets to elicit an ICMP Time Exceeded
message from each gateway to a particular host.
Using traceRoute The system provides two traceRoute functions:
■
traceRoute — Uses the hostname or IP address to trace a route to a
host with default options
■
advancedTraceRoute — Uses the hostname or IP address to trace a
route to a host with the advanced traceRoute options that you specify
traceRoute Operation To track the route of an IP packet, the traceRoute feature launches UDP
probe packets with a small TTL value and then listens for an ICMP Time
Exceeded reply from a gateway. Probes start with a small TTL of 1 and
increase the value by 1 until one of the following events occurs:
■
The system receives a Port Unreachable message, indicating that the
packet reached the host.
■
The probe exceeds the maximum number of hops. The default is 30.
At each TTL setting, the system launches three UDP probe packets, and
the traceRoute display shows a line with the TTL value, the address of the
gateway, and the round-trip time of each probe. If a probe answers from
different gateways, the traceRoute feature prints the address of each
responding system. If no response occurs in the 3-second time-out
interval, traceRoute displays an asterisk (*) for that probe. Other
characters that can be displayed include the following:
■
!N
— Network is unreachable
■
!H
— Host is unreachable
■
!P
— Protocol is unreachable
■
!F
— Fragmentation is needed
■
!< n>
— Unknown packet type