Foundation Summary

The Foundation Summary is a collection of information that provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary could help you recall a few details. If you just read this chapter, this review can help solidify some key facts. If you are doing your final preparation before the exam, these tables and figures are a convenient way to review the day before the exam.

With so many similar and mutually exclusive STP protection features available, you might have a hard time remembering which ones to use where. Use Figure 11-1 as a quick reference.

Figure 11-1. Guidelines for Applying STP Protection Features in a Network

Figure 11-1 shows two backbone switches (Catalyst A and B), along with an access-layer switch (Catalyst C), with redundant uplinks. Users are connected to the access switch, where PortFast is in use. An additional access switch (Catalyst D) has an uplink to access-layer switch C. All switch-to-switch links are fiber-based Gigabit Ethernet. Obviously, a Root Bridge never should appear out of Catalyst D.

Table 11-3. STP Protection Configuration Commands

Task	Global Command Syntax	Interface Command Syntax
Enable root guard		Switch(config-if)# spanning-tree guard root
Enable BPDU guard	Switch(config)# spanning-tree portfast bpduguard default	Switch(config-if)# spanning-tree bpduguard enable
Enable loop guard	Switch(config)# spanning-tree loopguard default	Switch(config-if)# spanning-tree guard loop
Enable UDLD	Switch(config)# udld {enable | aggressive | message time seconds}	Switch(config-if)# udld {enable | aggressive | disable}
Enable BPDU filtering	Switch(config)# spanning-tree bpdufilter default	Switch(config-if)# spanning-tree bpdufilter enable

Table 11-4. STP Protection Activity Commands

Task	Command Syntax
Look for ports that have been put in an inconsistent state	Switch# show spanning-tree inconsistentports
Display the global BPDU guard, BPDU filter, and loop guard states	Switch# show spanning-tree summary
Show UDLD status	Switch# show udld [type mod/num]
Re-enable all ports that UDLD has errdisabled	Switch# udld reset