When a bridge or switch using the Spanning-Tree Protocol first initializes, who does it claim ...

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“Do I Know This Already?” Quiz



125



You can find the answers to the “Do I Know This Already?” quiz in Appendix B on page 555.

Review the answers, grade your quiz, and choose an appropriate next step in this chapter based

on the suggestions in the “How to Best Use this Chapter” topic earlier in this chapter. Your

choices for the next step are as follows:



•

•



5 or fewer correct—Read this chapter.



•



9 or more correct—If you want more review on these topics, skip to the exercises at the

end of this chapter. If you do not want more review on these topics, skip this chapter.



6, 7, or 8 correct—Review this chapter, looking at the charts and diagrams that

summarize most of the concepts and facts in this chapter.



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Chapter 4: Understanding LANs and LAN Switching



Foundation Topics

LAN Overview

CCNA Objectives Covered in This Section

51



Describe full- and half- duplex Ethernet operation.



52



Describe network congestion problem in Ethernet networks.



55



Describe the features and benefits of Fast Ethernet.



56



Describe the guidelines and distance limitations of Fast Ethernet.



60



Define and describe the function of a MAC address.



This section provides some tables with important LAN details that you should memorize for

the exam. The section continues with details on Ethernet related to objectives 51, 52, 55, and 56.

The three main types of LANs that the CCNA exam covers are Ethernet, Token Ring, and

FDDI. There is a bias toward questions about Ethernet, which I think is reasonable given the

installed base in the marketplace. However, be prepared for questions on all three types.

The IEEE defines most of the standards for these three types of LANs. The summary

Table 4-1 lists the specification that defines the Media Access Control (MAC) and Logical Link

Control (LLC) sublayers.

Table 4-1 LAN Standards on the CCNA Exam

MAC Sublayer

Spec



LLC Sublayer

Spec



Ethernet Version 2

(DIX Ethernet)



Ethernet



Not applicable



This spec is owned by Digital,

Intel, and Xerox.



IEEE Ethernet



IEEE 802.3



IEEE 802.2



Also popularly called 802.3

Ethernet.



Token Ring



IEEE 802.5



IEEE 802.2



IBM helped development before

the IEEE took over.



FDDI



ANSI X3T9.5



IEEE 802.2



ANSI liked 802.2, so they just

refer to the IEEE spec.



Name



Other Comments



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127



The details of the LAN frames are shown in Figure 4-2. You should remember some details of

the contents of the headers and trailers for each LAN type—in particular, the addresses and their

location in the headers. Also, the name of the field that identifies the type of header that follows

the LAN headers is important. Finally, the fact that a frame check sequence (FCS) is in the

trailer for each protocol is also vital. Figure 4-2 summarizes the various header formats.

Figure 4-2



LAN Header Formats

8



IEEE

Ethernet



Presentation

6

6

2

1

Session

Preamble SD Dest. Address Source Address Length DSAP

Transport

7



1



Network



802.3



Network



Data Link

Data Link

1

1

1

6

6

1

Physical

Physical

IEEE

SD AC FC Dest. Address Source Address DSAP

Token Ring

Router 1

802.5

4

ANSI

FDDI



Preamble



1



1



1

SSAP



Host B

Application

Presentation

1-2 variable 4

1

Session

SSAP Control Data FCS

Transport

Network

802.2

Data Link

1-2 variable 4

Physical

Control Data FCS



802.2

6



6



SD FC Dest. Address Source Address



FDDI MAC



802.3

1



1



ED



FS



NA2603q3



Ethernet

(DIX)



6

6

2 variable

4

Host A

Preamble Dest. Address Source Address Type Data FCS

Application



802.5



1



1



1-2



variable



4



.5



1.5



DSAP



SSAP



Control



Data



FCS



ED



FS



802.2



FDDI



Largest Frame Sizes, excluding preambles, are:

Ethernet: 1518 bytes

Token Ring: 4472 bytes (4 MB)

17,800 bytes (16 MB)

FDDI: 4472 bytes (4 MB)



The function of identifying the header that follows the LAN header is covered rather

extensively in Chapter 3, “Understanding the OSI Reference Model.” Any computer receiving

a LAN frame needs to know what is in the “data” portion of the frame. (Refer to Figure 4-2 for

the data field.) Table 4-2 summarizes the fields that are used for identifying the types of data

contained in a frame.



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Chapter 4: Understanding LANs and LAN Switching



Table 4-2 Protocol Type Fields in LAN Headers

Field Name



Length



LAN Type



Comments



Ethernet Type



2 bytes



Ethernet



RFC 1700 (assigned Numbers RFC) lists the

values. Xerox owns the assignment process.



802.2 DSAP and

SSAP



1 byte each



IEEE Ethernet,

IEEE Token

Ring, ANSI

FDDI



The IEEE Registration Authority controls

the assignment of valid values. The source

SAP and destination SAP do not have to be

equal, so 802.2 calls for the sender’s

protocol type (SAP) and the destination’s

type.



SNAP Protocol



2 bytes



IEEE Ethernet,

IEEE Token

Ring, ANSI

FDDI



Uses EtherType values. Used only when

DSAP is hex AA. It is needed because the

DSAP and SSAP fields are only 1 byte in

length.



MAC Addresses

One important and obvious function of MAC addresses is to identify or address the LAN

interface cards on Ethernet, Token Ring, and FDDI LANs. These addresses are called unicast

addresses or individual addresses because they identify an individual LAN interface card. The

term unicast was chosen mainly for a contrast with the terms broadcast, multicast, and group

addresses. Frames between a pair of LAN stations use a source and destination address field to

identify each other.

Having globally unique Unicast MAC addresses on all LAN cards is a goal of the IEEE, so they

administer a program in which manufacturers encode the MAC address onto the LAN card,

usually in a ROM chip. The first half of the address is a code that identifies the vendor; the

second part is simply a unique number common to all cards that vendor has manufactured.

These addresses are called burned-in addresses (BIAs), and sometimes called Universally

Administered Addresses (UAA). The value used by the card can be overridden via configuration;

the overriding address is called a Locally Administered Address (LAA).

Another important function of IEEE MAC addresses is to address more than one LAN card.

Group addresses (as opposed to unicast or individual addresses) can address more than one

device on a LAN. This function is satisfied by three types of IEEE group MAC addresses:



•



Broadcast addresses—The most popular type of IEEE MAC address, the broadcast

address has a value of FFFF.FFFF.FFFF (Hex). The broadcast address implies that all

devices on the LAN should process the frame.



•



Multicast addresses—Used by Ethernet and FDDI, multicast addresses fulfill the

requirement to address a subset of all the devices on a LAN. Multicast addresses address

some subset of all the stations on the LAN. A station processes a frame to a particular



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129



multicast address only if configured to do so. An example of multicast addresses is a range

of addresses—1000.5exx.xxxx—where different values are assigned in the last three

bytes. Multicast addresses are also used in networks that implement IP multicast.



•



Functional addresses—Valid only on Token Ring, functional addresses identify one or

more interfaces that provide a particular function, for example, the Ring Error Monitor

function. An example is c000.0000.0001, which is used by the Token Ring Active

Monitor.



Finally, the order of bits in each byte of the addresses is different between Ethernet and the other

LAN types. As Figure 4-3 illustrates, the bytes are listed in the same order; however, the bit

order in each byte is opposite.

MAC Address Format

Vendor Code

(24 bits)

MAC

Address



Most

Significant

Byte



Least

Significant

Byte



Ethernet - Most Significant Bit is last

Token Ring and FDDI - Most Significant

Bit is first



NA260403



Figure 4-3



The bit order in Ethernet is called little-endian and on FDDI and Token Ring it is called bigendian. The meaning of these terms is that on Ethernet, the most significant bit in a byte is listed

last in the byte. For example, assume the binary string 01010101 is the value in a byte of an

Ethernet address. The right-most bit is considered to be the most-significant bit in this byte. The

hexadecimal equivalent is 55. However, if writing the same value in a byte of a Token Ring

address, the value written would be 10101010, so that the most significant bit is on the left, and

the hexadecimal equivalent would be AA. For example, the Token Ring address

4000.3745.0001 would be converted to 0200.ECA2.0080.

The following list summarizes many of the key features of MAC addresses:



•

•

•

•



Unicast MAC addresses address an individual LAN interface card.

Broadcast MAC addresses address all devices on a LAN.

Multicast MAC addresses address a subset of the devices on an Ethernet or FDDI LAN.

Functional MAC addresses identify devices performing a specific IEEE defined function,

on Token Ring only.



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Chapter 4: Understanding LANs and LAN Switching



•



Ethernet orders the bits in each byte of the MAC address with the least significant bit first;

this convention is called little-endian.



•



Token Ring and FDDI order the bits in each byte of the MAC address with the most

significant bit first; this convention is called big-endian.



•



The most significant bit on the first byte of an address must have a value of binary 0 for

unicast addresses, and 1 for broadcast, multicast, and functional addresses. This bit is

called the broadcast bit.



•



The second most significant bit in the first byte of the MAC address is called the local/

universal bit. A binary value of 0 implies that a burned-in or Universally Administered

Address (UAA) is being used; a binary 1 implies that a Locally Administered Address

(LAA) is being used.



Ethernet Standards and Operation

Several of the CCNA objectives (51, 52, 55, and 56) refer specifically to details of Ethernet

operation. This section covers the details relating to the CCNA objectives, as well as some

additional background. Equivalent details on Token Ring and FDDI are not covered here. Many

good sources exist for more information on Token Ring and FDDI, but you may want to refer

to your Cisco coursebooks or to Cisco Press’s Introduction to Cisco Router Configuration.

Table 4-3 lists the key Ethernet specifications and several related details about the operation

of each.

Table 4-3 Ethernet Standards



MAC Sublayer

Specification



Device

Connects to a

Hub or Directly

to a Bus



50 Ohm thick

coaxial cable



802.3



Bus



185 m1



50 Ohm thin

coaxial cable



802.3



Bus



100 m 2



UTP



802.3



Hub



Standard



Maximum Cable

Length



10B5



500 m1



10B2

10BT

10BFL

100BTx

100BT4

100BFx



2000



m2



Type of Cable



Fiber



802.3



Hub



100



m2



UTP/STP



802.3



Hub



100



m2



UTP, 4 pair



802.3



Hub



400



m2



Fiber



802.3



Hub



1. For entire bus

2. From device to hub/switch



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131



Ethernet congestion is most obvious when considering the 10B5 and 10B2 specifications. The

bus is shared between all devices on the Ethernet, using the carrier sense multiple access with

collision detection (CSMA/CD) algorithm for accessing the bus. (The bus also allows

transmission at 10 Mbps.) Basically, the following three features contribute to Ethernet

congestion:



•

•



Devices might have to wait before sending a frame if another frame is being received at

the same time that the device is ready to send. This increases latency while waiting for the

incoming frame to complete.



•



NOTE



Collisions could occur with normal use of the CSMA/CD algorithm if stations send

frames at (practically) the same instant in time. All collided frames sent are not received

correctly, so each station has to resend the frames. This wastes time on the bus.



There is a limit to the amount of bits that can be sent. The theoretical maximum

throughput for the LAN segment is 10 Mbps. For example, if the average frame is 1250

bytes, then 1000 frames per second would fill the Ethernet to its complete 10 Mbps

capacity.



As a reminder, the CSMA/CD algorithm works like this: The sender is ready to send a frame.

The device listens to hear if any frame is currently being received. When the Ethernet is silent,

the device begins sending the frame. During this time, the device listens (on the receiving pair)

because the frame it is sending is looped back onto its receive path. If no collisions occur, the

bits of the sent frame are received back successfully. If a collision has occurred, the collision is

detected because the received signal does not match the transmitted signal. In that case, the

device sends a jam signal then waits a random amount of time and repeats the process,

beginning with listening to hear if another frame is currently being received.



Full- and Half-Duplex Ethernet Operation

The use of full-duplex Ethernet can relieve some of the congestion. Half- and full-duplex

Ethernet imply the use of 10BT or some other hub-based topology.

Ethernet hubs were created with the advent of 10BT. These hubs are essentially multiport

repeaters; repeaters extend the bus concept of 10B2 and 10B5 by regenerating the same

electrical signal sent by the original sender of the frame. Therefore, collisions can still occur,

so CSMA/CD access rules continue to be used. Knowledge of the operation of Ethernet cards

and the attached hub is important to a complete understanding of the congestion problems and

a need for full-duplex Ethernet. Figure 4-4 outlines the operation of half-duplex 10BT with

hubs.



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Chapter 4: Understanding LANs and LAN Switching



Figure 4-4



10BT Half-Duplex Operation

Hub



5



NIC

Receive

Collision?

1



Loop

back



2-Pair Cable



4



Receive Pair

2

Transmit Pair



3



Transmit

NIC



4



5



NIC



5



NIC



NA260404



132



The chronological steps illustrated in Figure 4-4 are as follows:

1. The network interface card (NIC) sends a frame.

2. The NIC loops the sent frame onto its receive pair.

3. The hub receives the frame.

4. The hub sends the frame across an internal bus so all other NICs can receive the electrical



signal.

5. The hub repeats the signal out of each receive pair to all other devices.



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133



Because CSMA/CD rules are used when collisions could occur, full-duplex operation would

not be useful. If a card is receiving a frame, it would not choose to also start sending another

frame. Half-duplex operation is a side effect of the original design choice of retaining the

CSMA/CD media access for 10BT networks.

Full-duplex operation creates a situation whereby frames that are sent cannot collide with

frames being received. Imagine the use of Ethernet between a pair of NICs instead of cabling

the NIC to a hub. Figure 4-5 shows the full-duplex circuitry.

10BT Full-Duplex Operation

Receive



Receive



Transmit



Transmit



Full-Duplex NIC



Full-Duplex NIC



NA260405



Figure 4-5



Because no collisions are possible, the sender does not need to loop frames onto the receive

pair, as shown in Figure 4-5. Both ends can send and receive simultaneously. This reduces

Ethernet congestion related to all three points previously listed:



•

•



Collisions do not occur; therefore, time is not wasted retransmitting frames.



•



There are 10 Mbps in each direction, increasing the available capacity (bandwidth).



Waiting for others to send their frames is not necessary because there is only one sender

for each twisted pair.



Fast Ethernet

Fast Ethernet relieves congestion in some fairly obvious ways. Collisions and wait time are

decreased when compared to 10 Mbps Ethernet, simply because it takes 90 percent less time to

transmit the same frames. Capacity is greatly increased as well—with 1250 byte frames, a one

million frames per second theoretical maximum can be reached.

The two main features of Fast Ethernet are faster speed and autonegotiation. Autonegotiation

allows an Ethernet card, hub, or switch to determine which type of 100 Mbps Ethernet is

supported by the device/hub/switch on the other end of the cable. Also, support for half-duplex

or full-duplex is negotiated. And if the other device, such as a 10BT NIC, does not support

autonegotiation, autonegotiation will settle for half-duplex 10BT, assuming no overriding

configuration was added.

Table 4-4 outlines the Fast Ethernet specifications and a few details about cabling restrictions.