The
traceroute
privileged EXEC 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 an Internet Control Message Protocol (ICMP) time-to-live-exceeded message
to the sender. Traceroute finds 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 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-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).
To learn 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 datagram
destined to itself 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 that this message was sent by the destination port.
Go to
Example: Performing a Traceroute to an IP Host, on page 294
to see an example of IP traceroute process.
Debug Commands
Because debugging output is assigned high priority in the CPU process, it can render the system unusable.
For this reason, use
debug
commands only to troubleshoot specific problems or during troubleshooting sessions
with Cisco technical support staff. It is best to use
debug
commands during periods of lower network traffic
and fewer users. Debugging during these periods decreases the likelihood that increased
debug
command
processing overhead will affect system use.
Caution
All
debug
commands are entered in privileged EXEC mode, and most
debug
commands take no arguments.
System Report
System reports or crashinfo files save information that helps Cisco technical support representatives to debug
problems that caused the Cisco IOS image to fail (crash). It is necessary to quickly and reliably collect critical
crash information with high fidelity and integrity. Further, it is necessary to collect this information and bundle
it in a way that it can be associated or identified with a specific crash occurrence.
System reports are generated in these situations:
• In case of a switch failure—A system report is generated on the switch that failed
• In case of a switchover—System reports are generated only on high availability (HA) member switches.
reports are not generated for non-HA members.
The system does not generate reports in case of a reload.
During a process crash, the following is collected locally from the switch:
1.
Full process core
2.
Tracelogs
System Management Configuration Guide, Cisco IOS XE Fuji 16.8.x (Catalyst 9500 Switches)
270
Troubleshooting the Software Configuration
Debug Commands
