This chapter covers the following topics:
- Troubleshooting Switch Performance Issues: This section identifies common reasons why a switch might not be performing as expected.
- Troubleshooting Router Performance Issues: This section identifies common reasons why a router might not be performing as expected.
Switches and routers consist of many different components. For example, they contain a processor, memory (volatile such as RAM and nonvolatile such as NVRAM and flash), and various interfaces. They are also responsible for performing many different tasks, such as routing, switching, and building all the necessary tables and structures needed to perform various tasks.
The building of the tables and structures is done by the CPU. The storage of these tables and structures is in some form of memory. The routers and switches forward traffic from one interface to another interface based on these tables and structures. Therefore, if a router’s or switch’s CPU is constantly experiencing high utilization, the memory is overloaded, or the interface buffers are full, these devices will experience performance issues.
This chapter discusses common reasons for high CPU and memory utilization on routers and switches, in addition to how we can recognize them. This chapter also covers interface statistics because they sometimes provide the initial indication of some type of issue.
“Do I Know This Already?” Quiz
The “Do I Know This Already?” quiz allows you to assess whether you should read this entire chapter thoroughly or jump to the “Exam Preparation Tasks” section. If you are in doubt about your answers to these questions or your own assessment of your knowledge of the topics, read the entire chapter. Table 3-1 lists the major headings in this chapter and their corresponding “Do I Know This Already?” quiz questions. You can find the answers in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes.”
Table 3-1 “Do I Know This Already?” Section-to-Question Mapping
Foundation Topics Sectio
Troubleshooting Switch Performance Issues
Troubleshooting Router Performance Issues
What are the components of a switch’s control plane? (Choose two.)
- Forwarding logic
What are good indications that you have a duplex mismatch? (Choose two.)
- The half-duplex side of the connection has a high number of FCS errors.
- The full-duplex side of the connection has a high number of FCS errors.
- The half-duplex side of the connection has a high number of late collisions.
- The full-duplex side of the connection has a high number of late collisions.
Which of the following are situations when a switch’s TCAM would punt a packet to the switch’s CPU? (Choose the three best answers.)
- OSPF sends a multicast routing update.
- An administrator telnets to a switch.
- An ACL is applied to a switchport.
- A switch’s TCAM has reached capacity.
The output of a show processes cpu command on a switch displays the following in the first line of the output:
CPU utilization for five seconds: 10%/7%; one minute: 12%; five minutes: 6%
Based on the output, what percent of the switch’s CPU is being consumed with interrupts?
- 10 percent
- 7 percent
- 12 percent
- 6 percent
Which router process is in charge of handling interface state changes?
- TCP Timer process
- IP Background process
- Net Background process
- ARP Input process
Which of the following is the least efficient (that is, the most CPU intensive) of a router’s packet-switching modes?
- Fast switching
- Optimum switching
- Process switching
What command is used to display the contents of a router’s FIB?
- show ip cache
- show processes cpu
- show ip route
- show ip cef
Identify common reasons that a router displays a MALLOCFAIL error. (Choose the two best answers.)
- Cisco IOS bug
- Security issue
- QoS issue
- BGP filtering