General DataComm SpectraComm IP T1, Руководство по установке и эксплуатации

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Diagnostics & Monitoring Diagnostics Overview
6-4 SpectraComm IP T1 Router Card 076R200-000
Installation and Operation Manual Issue 13
Traceroute
Traceroute, invoked by the CLI command
trace-rte
, attempts to trace the route (path) a packet
takes through a network by making use of the TTL (Time to live) field in the IP packet header. The
TTL field is designed to prevent packets from running in loops by allowing every router that
forwards an IP packet to decrements the TTL field. When this field is decremented to zero, the
packet's life is expired. The router then discards the packet and sends an ICMP message indicating
Time Exceeded to the sender.
Traceroute sends a UDP datagram with a TTL value of 1 to the destination in an attempt to have the
first hop router identify itself by sending the ICMP Time Exceeded message. If successful,
traceroute then sends a UDP datagram with a TTL value of 2 to the destination attempting to
identify the second hop router, and so on until the destination is reached.
Figure 6-5 Trace Route
Special Considerations
TCP/IP was not designed to specifically support Traceroute. Therefore, Traceroute may not work
in all cases, but it is a useful technique for diagnosing IP connectivity problems. The following
conditions may affect Traceroute output:
• SCIPs in LANX mode, bridges and Etherswitches do not act as routers; they operate only in
layer 2 and are transparent to IP. Since these devices will not decrement the TTL field, they will
not show up in Traceroute output as a router hop.
• Traceroute traces the path of many packets, not just one. Ideally, all those packets would follow
the same route, but this is by no means assured. In the event of a link failure during the
Traceroute, your packets may be rerouted, resulting in a Traceroute output that is a confused
combination of two separate routes.
• You will only see one receiving IP address from each router, no sending addresses. That is, you
will see the interfaces routers are receiving packets on, but not the interfaces routers are sending
the packets on. The sending interfaces can often be deduced by matching each router with the
next one in line - typically only one interface would be used between them.
• Problems inherent in TCP/IP routing or implementation may result in packets with incorrect
TLL values, no route back to the sender, or a route through an interface other than the one the
packet was received on.
SCIP-T1% trace-rte
Enter destination IP address (a.b.c.d): 172.16.10.193
traceroute to 172.16.10.193 (30 hops max, 40 byte packets)
traceroute reply from 172.16.10.193: 7.050ms 9.600ms 9.750ms
trace-rte complete