62 Chapter 2: Spanning Tree Protocol

Foundation Summary

The “Foundation Summary” section lists the most important facts from the chapter. Although this section does not list everything that will be on the exam, a well-prepared CCNA candidate should at a minimum know all the details in each Foundation Summary before taking the exam.

Table 2-9 summarizes the reasons why spanning tree places a port in forwarding or blocking state.

Table 2-10 lists the default port costs defined by IEEE; Cisco uses these same defaults.

Table 2-10 Default Port Costs According to IEEE

Foundation Summary 63

Table 2-11 summarizes spanning tree’s intermediate states.

Table 2-12 lists the various RSTP and STP states.

Table 2-12 RSTP and STP Port States

64 Chapter 2: Spanning Tree Protocol

Foundation Summary

Spanning trees accomplish the goal of allowing physical redundancy, but with only one currently active path through a bridged network. Spanning tree uses the following features to accomplish the goal:

■All bridge interfaces eventually stabilize at either forwarding or blocking state. The forwarding interfaces are considered a part of the spanning tree.

■One of the bridges is elected as root. The election process includes all bridges claiming to be the root, until one bridge is considered best by all. All root bridge interfaces are in forwarding state.

■Each bridge receives hello BPDUs from the root, either directly or forwarded by another bridge. Each bridge can receive more than one such message on its interfaces. The port on which the least-cost BPDU is received is called the bridge’s root port. That port is placed in forwarding state.

■For each LAN segment, one bridge sends the forwarded BPDU with the lowest cost. That bridge is the designated bridge for that segment. That bridge’s interface on that segment is placed in forwarding state.

■All other interfaces are placed in blocking state.

■The root sends BPDUs every hello time seconds. The other bridges expect to receive copies of these BPDUs so that they know that nothing has changed. The hello time is defined in the BPDU itself, so all bridges use the same value.

■If a bridge does not receive a BPDU for MaxAge seconds, it begins the process of causing the spanning tree to change. The reaction can vary from topology to topology. (MaxAge is defined in the BPDU itself, so all bridges use the same value.)

■One or more bridges decide to change interfaces from blocking to forwarding, or vice versa, depending on the change in the network. When moving from blocking to forwarding, the interim listening state is entered first. After the Forward Delay amount of time (another timer defined in the root BPDU), the state is changed to learning. After another Forward Delay amount of time, the interface is placed in forwarding state.

■When a switch first transitions to a listening state, the switch sends a TCN BPDU over the new path to the root, forcing switches to quickly remove invalid entries from their MAC address tables.

The Spanning Tree Protocol includes these delays to help ensure that no temporary loops occur.

Q&A 65

Q&A

As mentioned in the Introduction, you have two choices for review questions. The following questions give you a bigger challenge than the exam because they are open-ended. By reviewing with this more-difficult question format, you can exercise your memory better and prove your conceptual and factual knowledge of the topics covered in this chapter. The answers to these questions are found in Appendix A.

For more practice with exam-like question formats, including multiple-choice questions and those using a router simulator, use the exam engine on the CD.

1.What routing protocol does a transparent bridge use to learn about Layer 3 address groupings?

2.What settings does a bridge or switch examine to determine which should be elected as root of the spanning tree?

3.If a switch hears three different hello BPDUs from three different neighbors on three different interfaces, and if all three specify that Bridge 1 is the root, how does the switch choose which interface is its root port?

4.Can the root bridge/switch ports be placed in blocking state?

5.Describe the benefits of Spanning Tree Protocol as used by transparent bridges and switches.

6.When a bridge or switch using Spanning Tree Protocol first initializes, what does it assert should be the tree’s root?

7.Name the three reasons why a port is placed in forwarding state as a result of spanning tree.

8.Name the three interface states that Spanning Tree Protocol uses, other than forwarding.

Which of these states is transitory?

9.What are the two reasons that a nonroot bridge/switch places a port in forwarding state?

10.Which two 2950 series EXEC commands list information about an interface’s spanningtree state?

This chapter covers the following subjects:

■Review of Virtual LAN Concepts

■Trunking with ISL and 802.1Q

■VLAN Trunking Protocol (VTP)

■VLAN and Trunking Configuration