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 |