Advanced STP Tuning

This chapter covers the following subjects:

• STP Topology Tuning: This section explains some of the options for modifying the root bridge location or moving blocking ports to designated ports.

• Additional STP Protection Mechanisms: This section examines protection mechanisms such as root guard, BPDU guard, and STP loop guard.

This chapter reviews techniques for configuring a switch to be guaranteed as the root bridge or as a backup root bridge for a Layer 2 topology. In addition, this chapter explains features that prevent other switches from unintentionally taking over the root bridge role. The chapter also explains other common features that are used in Cisco’s enterprise campus validated design guides.

“Do I Know This Already?” Quiz

The “Do I Know This Already?” quiz allows you to assess whether you should read the entire chapter. If you miss no more than one of these self-assessment questions, you might want to move ahead to the “Exam Preparation Tasks” section. Table 3-1 lists the major headings in this chapter and the “Do I Know This Already?” quiz questions covering the material in those headings so you can assess your knowledge of these specific areas. The answers to the “Do I Know This Already?” quiz appear in Appendix A, “Answers to the ‘Do I Know This Already?’ Quiz Questions.”

Table 3-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping

• 1. A switch’s STP priority can be configured in increments of ______.

  1. 1

  2. 256

  3. 2048

  4. 4096

• 2. True or false: The advertised path cost includes the advertising link’s port cost as part of the configuration BPDU advertisement.

  1. True

  2. False

• 3. True or false: The switch port with the lower STP port priority is more preferred.

  1. True

  2. False

• 4. What happens to a switch port when a BPDU is received on it when BPDU guard is enabled on that port?

  1. A message syslog is generated, and the BPDU is filtered.

  2. A syslog message is not generated, and the BPDU is filtered.

  3. A syslog message is generated, and the port is sent back to a listening state.

  4. A syslog message is generated, and the port is shut down.

• 5. Enabling root guard on a switch port does what?

  1. Upon receipt of an inferior BPDU, the port is shut down.

  2. Upon receipt of a superior BPDU, the port is shut down.

  3. Upon receipt of an inferior BPDU, the BPDU is filtered.

  4. When the root port is shut down, only authorized designated ports can become root ports.

• 6. UDLD solves the problem of ______.

  1. time for Layer 2 convergence

  2. a cable sending traffic in only one direction

  3. corrupt BPDU packets

  4. flapping network links

Answers to the “Do I Know This Already?” quiz:

• 1 D

• 2 B

• 3 A

• 4 D

• 5 B

• 6 B

Foundation Topics