11, SW2'S 0/27 INTERFACE BECAME THE DESIGNATED PORT ON THE SEGMENT B...

9-11, SW2's 0/27 interface became the designated port on the segment between SW2 and SW3.

The Need for Spanning Tree 251

STP places all other ports into a blocking state. In Figure 9-11, the only port that had not

been placed into a forwarding state was SW3's 0/27 interface, so it was placed into a

blocking state.

Table 9-5 summarizes the reasons why STP places a port in forwarding or blocking state.

Table 9-5

STP: Reasons for Forwarding State

Characterization of Port

Explanation

All root bridge’s ports

The root bridge is always the designated bridge on all

connected segments.

Each nonroot bridge’s root port

The root port is the port that receives the lowest-cost

BPDU from the root.

Each LAN’s designated port

The bridge that forwards the lowest-cost BPDU onto the

segment is the designated bridge for that segment.

All other ports

All ports that do not meet the other criteria are placed into

a blocking state.

STP uses a couple of port states besides forwarding and blocking.

■

Listening—Listens to incoming Hello messages to ensure that there are no loops, but

does not forward traffic or learn MAC addresses on the interface. This is an interim state

between blocking and forwarding.

■

Learning—Still listens to BPDUs, plus learns MAC addresses from incoming frames. It

does not forward traffic. This is an interim state between blocking and forwarding.

■

Disabled—Administratively down.

Under normal operation, when a port needs to change from blocking to forwarding, it first

transitions to listening, then learning, and then forwarding. This process, with default timers,

takes around 50 seconds.

STP might seem a bit overwhelming at this point. You should key on the general concepts,

and the interface states, for the INTRO exam. Refer to Chapter 2, “Spanning Tree Protocol,”

of the CCNA ICND Exam Certification Guide for a detailed discussion on STP.

Foundation Summary

The “Foundation Summary” section of each chapter lists the most important facts from the

chapter. Although this section does not list every fact from the chapter that will be on your

CCNA exam, a well-prepared CCNA candidate should know, at a minimum, all the details

in each “Foundation Summary” section before going to take the exam.

Transparent bridges forward frames when necessary and do not forward when there is no need

to do so, thus reducing overhead. To accomplish this, transparent bridges perform three actions: