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 can help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final preparation before the exam, this information is a convenient way to review the day before the exam.

STP has a progression of states that each port moves through. Each state allows a port to do only certain functions, as shown in Table 9-5.

Table 9-5. STP States and Port Activity

STP State	The Port Can...	The Port Cannot...	Duration
Disabled		Send or receive data
Blocking	Receive BPDUs	Send or receive data or learn MAC addresses	Indefinite if loop has been detected
Listening	Send and receive BPDUs	Send or receive data or learn MAC addresses	Forward Delay timer (15 seconds)
Learning	Send and receive BPDUs and learn MAC addresses	Send or receive data	Forward Delay timer (15 seconds)
Forwarding	Send and receive BPDUs, learn MAC addresses, and send and receive data		Indefinite as long as port is up and loop is not detected

Table 9-6. Basic Spanning-Tree Operation

Task	Procedure
1. Elect Root Bridge.	Lowest Bridge ID
2. Select Root Port (one per switch).	Lowest Root Path Cost; if equal, use tie-breakers
3. Select Designated Port (one per segment).	Lowest Root Path Cost; if equal, use tie-breakers
4. Block ports with loops.	Block ports that are non-Root and nonDesignated Ports

To manually work out a spanning-tree topology using a network diagram, follow the basic steps in Table 9-7.

Table 9-7. Manual STP Computation

Task	Description
1. Identify Path Costs on links.	For each link between switches, write the Path Cost that each switch uses for the link.
2. Identify Root Bridge.	Find the switch with the lowest Bridge ID; mark it on the drawing.
3. Select Root Ports (one per switch).	For each switch, find the one port that has the best path to the Root Bridge. This is the one with the lowest Root Path Cost. Mark the port with an RP label.
4. Select Designated Ports (one per segment).	For each link between switches, identify which end of the link will be the Designated Port. This is the one with the lowest Root Path Cost; if equal on both ends, use STP tie-breakers. Mark the port with a DP label.
5. Identify the blocking ports.	Every switch port that is neither a Root nor a Designated Port will be put into the Blocking state. Mark these with an X.

Table 9-8. Spanning-Tree Tie-Breaker Criteria

Sequence	Criteria
1	Lowest Root Bridge ID
2	Lowest Root Path Cost
3	Lowest Sender Bridge ID
4	Lowest Sender Port ID

Table 9-9. STP Path Cost

Link Bandwidth	STP Cost (Nonlinear Scale)
4 Mbps	250
10 Mbps	100
16 Mbps	62
45 Mbps	39
100 Mbps	19
155 Mbps	14
622 Mbps	6
1 Gbps	4
10 Gbps	2

Table 9-10. STP Timers

Timer	Function	Default Value
Hello	Interval between Configuration BPDUs.	2 seconds
Forward Delay	Time spent in Listening and Learning states before transitioning toward Forwarding state.	15 seconds
Max Age	Maximum length of time a BPDU can be stored without receiving an update. Timer expiration signals an indirect failure with Designated or Root Bridge.	20 seconds

Table 9-11. Types of STP

Type of STP	Function
CST	One instance of STP, over the native VLAN; 802.1Q based
PVST	One instance of STP per VLAN; Cisco ISL based
PVST+	Provides interoperability between CST and PVST; operates over both 802.1Q and ISL