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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 non–Designated 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

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