The following list describes common symptoms of high CPU utilization. If any
of these symptoms are noticed, follow the troubleshooting steps below to
alleviate the problem.
- High percentages in the show processes cpu command
output

- Input queue drops
- Slow performance
- Services on the router fail to respond, for instance:
- Slow response in Telnet or unable to telnet to the router
- Slow or no response to ping
- Router does not send routing updates
The output of a show processes cpu command from a
Cisco device, can be used by Output Interpreter to display potential issues and
fixes.
The show processes cpu command can be used to check if
CPU utilization is high due to interrupts. If it is not, check which process is
loading the CPU.
The main cause of CPU interrupts is the fast switching
of traffic. Interrupts are also generated any time a character is output from
the console or auxiliary ports of a router. However, Universal Asynchronous
Receiver/Transmitters (UARTs) are slow, compared to the processing speed of the
router, so it is not likely that console or auxiliary interrupts can cause a
high CPU utilization on the router.
There are several reasons for high
CPU utilization due to interrupts:
- Voice ports are configured on the router, even if there is no traffic,
software continues to monitor channel associated signaling (CAS).
- There are active ATM interfaces on the router, the ATM interfaces
continually send out null cells (per ATM standards) and continue to use CPU
resources.
- An inappropriate switching path is configured on the router.
- The CPU is performing memory alignment corrections, if %ALIGN-3-CORRECT
messages are logged, then the high CPU utilization is caused by memory
alignment corrections, which indicates that bugs in the version of the Cisco
IOS used.
If the router is overloaded with traffic, the show
interfaces
and
show interfaces switching commands provide information
about which interfaces are overloaded.
Output of the show
interfaces command can be used to examine the load and number of
throttles on interfaces. Throttles are a good indication of an overloaded
router. They show the number of times the receiver on the port has been
disabled, possibly due to buffer or processor overload. Together with high CPU
utilization on an interrupt level, throttles indicate that the router is
overloaded with traffic.
Output from the show interfaces
switching command can be used to see what kind of traffic, protocol,
and switching path, is going through the overloaded interface. If some
interfaces are too overloaded with traffic, consider redesigning the traffic
flow in the network or upgrading the hardware.
A single device may be
generating packets at an extremely high rate and overloading the router. In
this case the MAC address of that device can be isolated by adding the
ip accounting mac-address {input|output} interface
configuration command to the configuration of the overloaded interface. The
show interfaces [type number] mac-accounting or
show interfaces mac commands display the collected
information. Once the source device's MAC address is found, the
corresponding IP address can be found by checking the output of the
show arp privileged exec command.
Another possibility is that there might be a bug in the Cisco IOS Software
version running on the device. Once all the previous steps in this list have
been performed, check the
Bug Navigator, registered customers only, for a bug that reports similar
symptoms in a similar environment.