# CCNP Exam Prep: Traditional Spanning Tree Protocol

• Print
This chapter is from the book

## 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 9-6. 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