Cisco CCNA Troubleshooting CPU

First determine if the problem is real or is there a perceived problem.  If determined to be real (e.g. performance is not as expected), start troubleshooting.  Try and isolate the problem to a particular link, device or component.  After you have narrowed down the problem to a specific area, propose a solution and test it.


•Performance problems can be defined by any of the following:
•Business requirements and expectations
•Technical requirements and expectations
•User requirements and expectations
•Troubleshooting Performance Problems include the following:
•Assess the problem
•Isolate the problem
•Resolving the problem


Cisco CCNA Troubleshooting Utilization

The main functions a router CPU performs are packet forwarding and executing control plane and management processes.  When high CPU utilization occurs it is important to understand when it is a potential problem and when temporary high CPU utilization might be normal.  High CPU utilization could be attributable to short network burst which would be considered normal or if CPU utilization is consistently high it will be impacting performance and needs to be investigated.

Cisco CCNA Troubleshooting Router Slowdown

When the CPU is too busy network performance suffers.  Things such as packet loss can occur as buffers fill up, jitter (variations in packet delay) as the CPU is not processing packets in a timely manner and higher than normal delays due to the same problem as just mentioned.

Cisco CCNA Troubleshooting

Numerous problems can arise due to high CPU utilization.  Some common problems are listed on the slide above.


•Slow or no response on SSH or Telnet sessions
•Slow or no response to ping
•Slow response on the console
•Router not sending routing updates to other routers or not receiving / processing routing updates from other routers


Cisco CCNA Troubleshooting CPU


Some of the common causes to high router CPU utilization are listed on the slide above.

•ARP Input process: Originates ARP requests
•IP Background process: Responsible for moving an interface to a new state (up or down), changing the encapsulation type and changes to the IP address on an interface
•TCP Timer process: Manages TCP sessions running on the router
•Net Background process: Creates buffers from the main buffer pool whenever required, but not available to the process or interface


Cisco CCNA Troubleshooting CPU

Listed on the slide above are just a handful of examples that can cause high router CPU utilization.


•There are numerous reasons for high CPU utilization.  Some examples are as follows:
•Router may originate an excessive number of ARP requests
•Excessive number of throttles, overruns and ignored packets on an interface
•Router having too many TCP peers
•Interface flapping (up and down)
•Routing table changes due to interface status change



Cisco CCNA Troubleshooting CPU

The show processes cpu command displays overall router CPU utilization as well as the CPU utilization of each of the processes running on the router. The output CPU utilization for the past 5 seconds, past 1 minute and past 5 minutes.

The show processes cpu history command is similar to the show processes cpu command although it displays router utilization for the past 1 minute, 60 minutes and 72 hours.

Cisco CCNA Troubleshooting Memory

In the example on the slide, in order to utilize the Advanced Enterprise Services version of IOS your 3800 series router would need to have a minimum of 256MB of DRAM and 64MB of Flash.


•The router does not have enough memory to support the Cisco IOS Software image:
•Prior to downloading an IOS image from Cisco’s web site it tells you what the memory requirements are