OTHERWISE, FILTER (DO NOT FORWARD) THE FRAME.THE INTERNAL PROCESSIN...

5.

Otherwise, filter (do not forward) the frame.

The internal processing algorithms used by switches vary among models and vendors;

regardless, the internal processing can be categorized as one of the methods listed in Table 9-6.

Foundation Summary 253

Table 9-6

Switch Internal Processing

Switching Method

Description

Store-and-forward

The switch fully receives all bits in the frame (store) before

forwarding the frame (forward). This allows the switch to check the

FCS before forwarding the frame. (The FCS is in the Ethernet trailer.)

Cut-through

The switch performs the address table lookup as soon as the

destination address field in the header is received. The first bits in the

frame can be sent out the outbound port before the final bits in the

incoming frame are received. This does not allow the switch to

discard frames that fail the FCS check. (The FCS is in the Ethernet

trailer.)

Fragment-free

This performs like cut-through switching, but the switch waits for 64

bytes to be received before forwarding the first bytes of the outgoing

frame. According to Ethernet specifications, collisions should be

detected during the first 64 bytes of the frame, so frames in error

because of a collision will not be forwarded.

General definitions for a collision domain and a broadcast domain are as follows:

■

A collision domain is a set of network interface cards (NICs) for which a frame sent by

one NIC could result in a collision with a frame sent by any other NIC in the same

collision domain.

■

A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC will

be received by all other NICs in the same broadcast domain.

Figure 9-12 shows a typical example of the definition of collision domains, while Figure 9-13

shows broadcast domains in the same network.

Figure 9-12

Collision Domains

Figure 9-13

Broadcast Domains

Table 9-7 summarizes the reasons STP places a port in forwarding or blocking state.

Table 9-7

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

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

port

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.

Q&A 255

Q&A

As mentioned in the introduction, you have two choices for review questions. The questions

that follow give you a bigger challenge than the exam itself by using an open-ended question

format. By reviewing now with this more difficult question format, you can exercise your

memory better and prove your conceptual and factual knowledge of this chapter. The

answers to these questions are found in Appendix A.

For more practice with exam-like question formats, including questions using a router

simulator and multiple-choice questions, use the exam engine on the CD.