Tài liệu Basic Networking Technologies docx

net-The following CCIE blueprint objectives as determined by the Cisco SystemsCCIE program are covered in this chapter: ➤ Data Link layer—MAC addressing and IEEE 802.2 standards ➤ Ethern

Terms you’ll need to understand:

✓ Media Access Control (MAC)

✓ Quality Of Service (QOS)

Techniques you’ll need to master:

✓ Describing layer 2 MAC addresses

✓ Working with Ethernet, Token

Ring, and FDDI characteristics andlimitations

✓ Understanding basic multiservicetheory

This chapter concentrates on the characteristics and limitations of the differenttypes of Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI)technologies After reviewing each of these technologies, the chapter briefly turns

to voice and video communications that can be delivered over existing data works These topics are called Multiservice Services by Cisco

net-The following CCIE blueprint objectives as determined by the Cisco SystemsCCIE program are covered in this chapter:

➤ Data Link layer—MAC addressing and IEEE 802.2 standards

➤ Ethernet/Fast Ethernet/Gigabit Ethernet—Encapsulation, Carrier Sense

Mul-tiple Access with Collision Detect (CSMA/CD), topology, speed, controllererrors, limitations, and the IEEE 802.3 standards

➤ Token Ring—Token passing, beaconing, active monitor, ring insertion, soft and

hard errors, topology, maximum transmission unit (MTU), speed, limitations

➤ FDDI/CDDI—Dual ring, encapsulation, class, redundancy, dual homing,

medium (including copper and fiber), claims, station management (SMT),limitations

➤ Voice/Video—H.323, codecs, Signaling System 7 (SS7), Realtime Transport

Protocol (RTP), RTP Control Protocol (RTCP), Quality Of Service (QOS)Additional information is provided for completeness and in preparation for addi-tional subjects as the CCIE program expands We will begin by discussing whatmakes up a MAC address

MAC Addressing

All devices that operate over a physical LAN medium require a unique address,called the Media Access Control (MAC) address The MAC address is also some-times referred to as the physical address, burned-in address (BIA), or hardwareaddress A MAC address is assigned to each hardware device that connects to aLAN, such as an Ethernet NIC In Token Ring networks, the MAC address can

be set in software

In IEEE 802 networks, the Data Link Control (DLC) layer of the OSI referencemodel is divided into two sublayers: the Logical Link Control (LLC) layer andthe Media Access Control (MAC) layer Figure 4.1 displays the location of theLLC sublayer and the MAC sublayer in relation to the OSI model

LLC Sublayer Functions

The LLC sublayer provides networks with connection or connectionlessenviroments The LLC sublayer simply sits on top of all 802.x protocols andprovides a service to the Network layer

Using IP as an example, we know that IP is a connectionless service, but the role

of the LLC sublayer is to identify that an IP packet is carried in the data portion

of the frame The IP software then looks further into the frame to locate theheader information and the IP address

MAC Sublayer Functions

The MAC sublayer simply provides access to the Physical layer, whether Ethernet

or any other medium is in use To allow this communcaition each device musthave a unique address

To enable all devices to have a unique address or MAC address, the networkinterface cards have a unique MAC address located in Read Only Memory(ROM) This unique address allows communication between devices regardless

of the physical medium Let’s now describe the format of the MAC address.MAC addresses are 48 bits long, and they are expressed primarily in two formats:

Network

Data Link

Physical

Logical Link Control

Media Access Control

Figure 4.1 IEEE 802 DLC.

Canonical vs Non-Canonical

The IEEE refers to MAC addresses as Universal Addresses The IEEE also specifiedthat when the bits are sent across the Ethernet Physical layer the least significantbit is transmitted first This is referred to as non-canonical Token Ring is canonical,which means that the most significant bit is transmiited first Let’s look at a simpleexample of sending the number 1 (decimal) across an Ethernet network The num-ber 1 in binary is 00000001 The non-canonical format of this is 10000000 Thismeans that on the Ethernet wire the bits 10000000 will be reversed by the receivingdevice back to 00000001

The majority of modern networks use a 48-bit addressing scheme or plan MACaddresses are represented using the hexadecimal numbering system The first 24bits represent the manufacturer’s identification, vendor’s code, or the organiza-tion unique identifier (OUI) The next 24 bits typically provide a serial numberassigned by the vendor To illustrate, here is an example of a Cisco MAC address:006070-155e4d

In the preceding address, 00-60-70 (24 bits) identifies Cisco as the manufacturer

or vendor code, and 15-5e-4d (24 bits) identifies the serial number assigned byCisco Manufacturers such as Cisco may have more than one OUI For instance,Cisco Systems has more than 20 OUIs from the IEEE

Frames sent to MAC addresses can be classified as being sent to either unicast,multicast, or broadcast addresses:

➤ Unicast Frame—A frame destined for a specific device In the destination

address, a unicast frame will appear as 0xxxxxxx in the first byte

➤ Multicast Address—A special address reserved for communication among a

group of devices For example, 1xxxxxxx in the first byte

➤ Broadcast Address—An address destined for all devices on the wire For

ex-ample, FF-FF-FF-FF-FF-FF in the destination field indicates all devicesmust read the frame

Note that all frames will have their source MAC address as a single node (unicast)

Ethernet, Fast Ethernet, and

Gigabit Ethernet

Ethernet is one of the most popular local area network (LAN) technologies usedtoday Ethernet can operate at three speeds:

➤ Ethernet—Allows transmission speeds of 10Mbps

➤ Fast Ethernet—Allows transmission speeds of 100Mbps

➤ Gigabit Ethernet—Allows transmission speeds of 1,000Mbps

10 Gigabit Ethernet is coming soon The networking industry has formed

a coalition to make 10G a reality

Originally, Ethernet started when the Xerox Corporation released a method ofallowing devices to share a common medium and communicate together Table 4.1shows a summary of Ethernet’s recent evolution

In this section, we’ll review the three main Ethernet types, starting with a sion about traditional Ethernet

discus-Ethernet 802.3 and discus-Ethernet_II

Ethernet has two versions available—Ethernet 802.3 and Ethernet_II The maindifference between Ethernet 802.3 and Ethernet_II can be found within theframe formats, as discussed later in this section Original Ethernet and then the

second version called Ethernet_II was jointly developed by the Digital

Equip-ment Corporation, Intel, and Xerox Corporation, also know as the DIX tium Ethernet 802.3 is the standard defined by the IEEE

Consor-The Ethernet specifications also define the frame sizes as follows:

➤ Minimum Ethernet Frame Size—64 bytes

➤ Maximum Ethernet Frame Size—1,514 bytes

Table 4.1 Ethernet history.

1972 Work begins on Ethernet by Xerox

1982 Version II released by DIX (Digital, Intel, and Xerox)

1985 IEEE 802.3 Ethernet Standard is released

1994 Transmission of Ethernet over twisted pair wiring is released

When running IP, Ethernet_II is the default transmission method forCisco Routers.

Both Ethernet 802.3 and Ethernet_II are shared physical media technologies.This means that when an end device on an Ethernet network needs to send data,

it first must wait to see if the physical medium is not being used before sion can commence The end device will listen to the wire using its ability tocarrier sense, which is part of the CSMA/CD (CS stands for carrier sense.)Further, while the data is transmitting, the end device must ensure no other de-vice has transmitted at the same time on its receive interface The sending stationwill also listen to see if received data is different from the transmitted data If it is

transmis-different, a collision occurred Specifically, the end device is listening for a change

in voltage on the wire

When a collision occurs, the sending end device transmits a jam signal (a random

signal used to inform all devices that a collision has been detected) and backs offfor a random value calculated with the back off algorithm This method is called

Collision Sense Multiple Access with Collision Detection (CSMA/CD) As a result

of Ethernet’s shared-medium properties, Ethernet is sometimes referred to as

undeterministic This is because end devices don’t know when they can send data

(that is, when the wire is clear), and end devices aren’t aware that another devicewill transmit at the same time A deterministic device is able to calculate themaximum time that will pass before any end station will be able to transmit

What is half-duplex and full duplex Ethernet? Half duplex Ethernet lows only one device to send data or receive data at a time Full duplexEthernet allows the capability of simultaneous data transmission be-tween two devices Full duplex Ethernet allows for better use of theavailable bandwidth because both devices can send and receive data

al-at the same time

To illustrate CSMA/CD in action, Figure 4.2 shows an Ethernet network withfour PCs (end devices) trying to communicate The following occurs:

1 PC-1 listens and determines that no one else is sending data

2 PC-1 starts to transmit information if the wire is clear At exactly the sametime, PC-2 decides to send data after also determining that no other devicewas using the media to send data

4 At some point, the bits will collide, a collision will be detected by the sion detection circuitry within each PC’s NIC, and both devices will send ajam signal and then back off for a random amount of time before attempting

colli-to retransmit

5 PC-1 sends data once more after completing Step 1 again This will be tried

up to 15 times before an error is sent to the user applications

When using a shared media, such as Ethernet, collisions are a part of Ethernet’soperation, and they are considered normal However, excessive collisions can causedelays and reduce available bandwidth to end devices Typically, after networkutilization goes above 12 percent, you will start to see excessive collisions Exces-sive collisions will result in time delays, and end user performance will be im-pacted When utilization reaches 30 percent, Ethernet networks will start toexperience longer delays and excessive collisions

As mentioned earlier, there are two types of Ethernet—Ethernet 802.3 andEthernet_II There are four frame type formats that are supported in Ethernet.The frame formats vary for each of these Ethernet types Figure 4.3 displays the

PC-1 has

data to send

PC-2 has data to send

PC-3 has no

Collision occurs

Figure 4.2 Using Ethernet devices to send data using CSMA/CD.

four Ethernet frame formats, which can contain the Ethernet_II, Ethernet 802.3,Ethernet 802.2, and the SNAP Ethernet frame.

Note: Previously, we mentioned that the minimum size of an Ethernet frame is 64

bytes Now that we have introduced the preamble, it is crucial to note that while the preamble is part of the Ethernet frame, it is not considered when determining size Thus, an Ethernet frame with a size of 64 bytes (minimum) or 1518 bytes

(maximum) has a preamble that is not counted in the frame size.

We will now cover what each field is responsible for in the four Ethernet frame types.Ethernet_II has the following frame format parameters:

➤ Preamble (8 bytes)—The preamble is used to synchronize all stations.

➤ Destination Address (6 bytes)—The destination address can be unicast

(spe-cific device), multicast (group of addresses), or broadcast (all devices)

➤ Source Address (6 bytes)—The source address identifies the sender.

➤ Type (2 bytes)—The type field describes the protocol been carried in the frame.

➤ Data (46 to 1,500 bytes)—The data field carries end user information, such as

an email message

➤ Frame Checksum (4 bytes)—This frame checks sequence and calculates all fields,

except the preamble and the frame checksum (FCS), to make sure the frame

Type 2

Data

46 to 1500

FCS 4 Ethernet 802.3 Frame

Length 2

802.2 or SNAP Header (see below)

46 to 1500

FCS 4 Ethernet 802.2 Frame

Length 2

802.2 Field DSAP

1

SSAP 1

CTRL 1

Data

46 to 1497

FCS 4 Ethernet SNAP Frame

Length 2

AA 1

AA 1

CTRL 1

Data

46 to 1496

Ethernet Type 1

FCS 4 SNAP Fields

Figure 4.3 Four Ethernet frame types.

The total length of an Ethernet frame must never exceed 1,518 bytes,

or a frame called a giant will be generated The smallest frame size is

64 bytes A frame smaller than 64 bytes is called a runt

Ethernet 802.3 has the following frame format parameters:

➤ Preamble (8 bytes)—The preamble is used to synchronize all stations.

➤ Destination Address (6 bytes)—The destination address can be unicast

(spe-cific device), multicast (group of addresses), or broadcast (all devices)

➤ Source Address (6 bytes)—The source address identifies the sender.

➤ Length (2 bytes)—The length field describes data length.

➤ Data (46 to 1,500 bytes)—The data field carries end user information, such as

an email message

➤ Frame Checksum (4 bytes)—This frame checks sequence and calculates all fields,

except the preamble and the frame checksum (FCS), to make sure the frame

is not corrupted

The main difference between Ethernet_II and Ethernet 802.3 is thatEthernet_II has a type field and 802.3 has a length field If the contents

of this field exceed a value of 1,518, devices will know they are in

possession of a Ethernet_II frame and read the field as a type field Ifthe value is less than 1,518, the field is treated as a length field

Ethernet 802.2 has the following frame format parameters:

➤ Preamble (8 bytes)—The preamble is used to synchronize all stations.

➤ Destination Address (6 bytes)—The destination address can be unicast

(spe-cific device), multicast (group of addresses), or broadcast (all devices)

➤ Source Address (6 bytes)—The source address identifies the sender.

➤ DSAP (1 byte)—The Destination Service Access Point field together with

the SSAP define the source and destination protocol of the frame

➤ SSAP (1 byte)—The Source Service Access Point field together with the DSAP

define the source and destination protocol of the frame

➤ CTRL (1 byte)—The control field.

➤ Data (46 to 1,497 bytes)—The data field carries end user information, such as

an email message

➤ Frame Checksum (4 bytes)—This frame checks sequence and calculates all fields,

except the preamble and the frame checksum (FCS), to make sure the frame

is not corrupted

Note: With an IPX packet, the 802.2 header is set to E0 E0 03.

Ethernet SNAP header has the following frame format parameters:

➤ Preamble (8 bytes)—The preamble is used to synchronize all stations.

➤ Destination Address (6 bytes)—The destination address can be unicast

(spe-cific device), multicast (group of addresses), or broadcast (all devices)

➤ Source Address (6 bytes)—The source address identifies the sender.

➤ DSAP (1 byte)—The Destination Service Access Point field together with

the SSAP define the source and destination protocol of the frame For a SNAPframe, this field is set to AA

➤ SSAP (1 byte)—The Source Service Access Point field together with the DSAP

define the source and destination protocol of the frame For a SNAP frame,this field is set to AA

➤ CTRL (1 byte)—The control field.

➤ Data (46 to 1,496 bytes)—The data field carries end user information, such as

an email message For a SNAP frame will include a type field that will tify the payload type like IP for example

iden-➤ Frame Checksum (4 bytes)—This frame checks sequence and calculates all fields,

except the preamble and the frame checksum (FCS), to make sure the frame

is not corrupted

In the Control Fields (CTRL), 03 indicates Logical Link Control Type I ordatagram service In Ethernet_II, the type field identifies the payload orend user data Some common type field examples include the following:

Table 4.2 displays three different physical Ethernet standards used in Ethernetnetworks The first two digits display the speed (in this case 10, it could be 100 or

1,000 for example), the word Base identifies this as baseband (BASE, where one

carrier frequency is used), and finally the last digit identifies the length and cable

type For example,10BaseT is 10Mb Ethernet, Baseband, Unshielded twisted pair

and a maximum length of 100m

Fast Ethernet

Fast Ethernet operates at 100Mbps Fast Ethernet’s frame format and MACaddresses are the same as Ethernet’s frame format and MAC addresses The majordifferences between Ethernet and Fast Ethernet is that Fast Ethernet can oper-ate over many types of physical layer connections, including four pair twisted paircable Another major difference is that Fast Ethernet, when cabled with twistedpair cable, has a maximum network diameter of only 205 meters Both Ethernetand Fast Ethernet use CSMA/CD to gain access to media Table 4.3 summa-rizes a number of standards for Fast Ethernet (802.3u)

Today, an even faster version of Ethernet is available, called Gigabit Ethernet.Gigabit Ethernet allows transmissions rates up to 1,000Mbps

Gigabit Ethernet

Gigabit Ethernet is a recent development that enables transmissions up to1,000Mbps Gigabit Ethernet is an extension of the IEEE 802.3 standard andwas developed to meet the needs of an industry that demands more bandwidth

as time goes by Gigabit Ethernet uses the same frame formats and MTU sizes,and uses the CSMA/CD algorithm as well 802.3z defines Gigabit Ethernet.(For more information about Gigabit Ethernet, visit the IEEE Web site at

www.ieee.com.)

Table 4.2 Ethernet (802.3) characteristics.

Ethernet Standard Characteristic

10BaseT 10Mbps over two pair twisted cable Maximum length 100m 10Base2 10Mbps over coaxial cable (RG58) Maximum length 185m.

10Base5 10Mbps over thick Ethernet Maximum length 500m.

Table 4.3 Three Fast Ethernet (802.3u) characteristics.

Ethernet Standard Characteristic

100Base-Tx 100Mbps over two pair twisted wire Maximum length 100m 100Base-T4 100Mbps over four pairs of category 3, 4, or 5 cable.

Maximum length 100m.

100Base-Fx 100Mbps over two strands of fiber Maximum length 400m.

Verifying Ethernet Operation

Now that we’ve talked a little bit about theory, let’s look at a Cisco router’s tical display on a 10Mbps Ethernet interface and review the fields you need to be

statis-aware of that are provided to you in a show interface command Listing 4.1

pro-vides a sample Ethernet statistical display taken from a Cisco router

Listing 4.1 The show interface Ethernet0 command.

Ethernet0 is up, line protocol is up

Hardware is Lance, address is 0000.0c92.2ed4

Internet address is 10.99.34.50/24

MTU 1500 bytes,BW 10000 Kbit,DLY 1000 usec,rely 255/255,load 1/255 Encapsulation ARPA, loopback not set, keepalive set (10 sec) ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters never

Queuing strategy: fifo

Output queue 0/40, 0 drops; input queue 0/75, 0 drops

5 minute input rate 2000 bits/sec, 2 packets/sec

5 minute output rate 1000 bits/sec, 2 packets/sec

533880 packets input, 74463913 bytes, 0 no buffer

Received 524894 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

0 input packets with dribble condition detected

94282 packets output, 8713055 bytes, 0 underruns

1 output errors, 141 collisions, 2 interface resets

0 babbles, 0 late collision, 230 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

The following list highlights the most important fields relative to ing and understanding how Ethernet is operating:

troubleshoot-➤ MTU—Maximum transmission unit.

➤ BW—Interface bandwidth, measured in Kbps.

➤ DLY—Interface delay, measured in microseconds.

➤ rely—Reliability of the interface; 255 out of 255 means the interface is 100

percent reliable

➤ load—Interface load; 255/255 means the interface is 100 percent loaded.

➤ ARP type—Type of Address Resolution Protocol assigned.

➤ packets input—Total number of error-free packets received by the system.

➤ bytes—Total number of bytes received by the interface (Note that this is on

the same line as packets input.)

➤ no buffers—Number of packets discarded because the router had no available

buffers to store the packet before delivery (Broadcast storms can typicallymake this number high, because the router might not be able to handle theamount of packets received.)

➤ Received broadcasts—Broadcast or multicast packets received by the interface.

➤ runts—Packets less than the minimum 64 bytes required for Ethernet.

➤ giants—Packets greater than the maximum allowable frame size in Ethernet.

Maximum frame size is set to 1,500 on Cisco router (MTU 1500)

➤ input errors—Number of runts, giants, no buffer available, CRC, frame,

over-run, and ignored counts

➤ CRC—Cyclic redundancy checksum Calculated by the source station and

checked by the router

➤ frame—Number of frames received that have incorrect checksum errors.

➤ overrun—Number of times the receiver hardware was unable to hand

re-ceived data to a hardware buffer because the input rate exceeded the receiver’sability to handle the data

➤ ignored—An internal condition on the router that indicates how many times

the interface runs low on internal buffers

➤ input packets—A dribble bit error that indicates that a frame is slightly

too long

➤ with dribble—Number of packets that have been seen by the router that are

slightly larger than the maximum frame size

➤ packets output—Total number of messages transmitted by the system.

➤ bytes—Total number of bytes put out by the interface.

➤ underruns—Number of times the transmitter (Tx) has run faster than the

router can handle

➤ output errors—Sum of all errors.

➤ collisions—Number of collisions detected by the router on the local Ethernet.

➤ interface resets—Number of times the interface has been reset Interface

re-sets can occur manually with the clear interface E0 command or due to an

error condition on the segment, such as excessive broadcasts

The preceding fields are important when troubleshooting Ethernet networks fromthe viewpoint of a Cisco router For example, an interface that reports a highnumber of collisions is indicative of a device that might be faulty on the network

Note: The interface command show interface fastethernet followed by the interface

number displays the same statistical display shown by the show interface Ethernet0

command The notable difference is the bandwidth parameter, which is set to

100000 Kbit as opposed to 10000 Kbit for Ethernet.

The overriding benefits of Ethernet are that it’s cheap and easy to install Further,with Gigabit Ethernet’s recent developments, the future looks good Now, let’sturn to the most common networking technology used in the late 1970s and

’80s—Token Ring

Token Ring 802.5

Token Ring networking was developed by IBM and Texas Instruments in the1970s in response to the growing popularity of the personal computer The IEEEcommittee defined Token Ring in 802.5 to provide a standard to be used by non-IBM devices

In a Token Ring network, a device must have possession of the token frame before

it can transmit data onto the ring Possession of the token frame allows a device

to send data onto the ring Without a token frame, devices are not permitted to

transmit Hence, Token Ring networks are sometimes called deterministic,

be-cause the possession of a free token determines whether a device can transmitacross a medium Figure 4.4 displays a typical scenario where a group of devicesattached to the ring must wait for a free token before data can be sent across thenetwork The free token is placed onto the ring once a device has finished send-ing data Token Ring networks are deterministic because each station has equalaccess to the token and, therefore, equal access to the network The token rotatesthrough the ring in a predictable fashion

In a Token Ring network, a station must wait for the token to be available before

it can send data on to the ring After a device possesses the token, the device canthen send data The data is circulated around the ring until the destination devicehas copied the frame and returned the frame into the ring Then, the sendingdevice must remove the frame from the ring and place the free token back ontothe ring The exception to this rule is in the case of early release, when the receiv-ing station can release the free token

Token Ring 802.5 can run at two speeds—4Mbps and 16Mbps To modify thering speed on a Cisco router, you use the following IOS command:

ring-speed <4 or 16>

In summary format, the main characteristics of Token Ring 802.5 are:

➤ Star topology

➤ Star cabling

➤ Data rates are 4Mbps or 16Mbps

➤ Logically a ring

➤ Full duplex modes are supported on Cisco routers only when there are justtwo stations on the ring Full Duplex Token Ring allows two devices to trans-mit simultaneously without the existence of a token

Note: Token Ring is phsically a star topology Therefore, Token Ring is sometimes

referred to as a star.

Now, let’s look at the Token Ring frame format defined in 802.5

Token Ring Frame Formats

There are two Token Ring frame formats defined in 802.5—a token frame with

no data and a frame that contains data (that is, a busy token) A token frame with

no data contains the following fields:

SD AC ED

Token Ring

Token frame

This PC has the free token

Possession of token permits data transfer

Figure 4.4 Token Ring data transfer.

A token data frame (a token carrying data) can contain the following fields:

SD AC FC DA SA RIF Data FCS ED FS

Notice that the Token Ring data frame format has more fields when compared to

an Ethernet frame This makes Token Ring a little more robust, as you can see inthe following descriptions:

➤ SD (Starting Delimiter, 1 byte)—Indicates the start of the frame and is

repre-sented as JK0JK000 in hexadecimal Don’t worry too much about this field,but it merely is used to indicate any Manchester code violations

➤ AC (Access Control, 1 byte)—Contains parameters that define the priority (1

bit that indicates whether the frame is a data or free token) A bit that

indi-cates if the active monitor has seen the frame The active monitor is a device

on the ring that maintains the ring The AC field is represented as

PPPTMRRR, where the P bits are priority bits, T bits identify whether the frame is a token or data frame, M identifies the monitor bit, and RRR speci-

fies the reservation bits

➤ ED (End Delimiter, 1 byte)—Indicates the end of the frame This field is set

to JK1JK11E

➤ FC (Frame Control, 1 byte)—Indicates which type of frame is arriving.

➤ DA (Destination Address, 48 bits)—Indicates the destination MAC address.

➤ SA (Source Address, 48 bits)—Indicates the source MAC address If the first

bit of the source address is set to 1, a routing information field (RIF) will bepresent

➤ RIF (Routing Information Field)—Describes the routing information field,

which can be up to 18 bytes in length

➤ Data (>0 bits)—Contains user data.

➤ FCS (Frame Check Sequence, 32 bits)—Checks the FC, DA, SA, and Data fields.

➤ FS (Frame Status, 8 bits)—Indicates if the frame was recognized by another

device and copied This field is represented as AC00AC00, where A is set to

1 when the address is recognized and C is set to 1 when the frame is copied The R bits are reserved The A and C bits are copied, because there is no

redundancy check or CRC made on this field

An important fact is that Token Ring supports two broadcasts frametypes—FF-FF-FF-FF-FF-FF and C0-00-FF-FF-FF-FF

Figure 4.5 illustrates a variety of bit combinations and serves as a very handydiagram for troubleshooting Token Ring networks.

Token Ring has a number of built-in stations that monitor and maintain thering These stations enable the ring to recover from faults and error conditions,such as when there are no free tokens circulating a ring for an extended time Thestations ensure that the token is always available and report problems to networkprotocol analyzers, if they are present Table 4.4 summarizes the functions per-formed by Token Ring stations

Token Frame (No data)

is a LLC (01) or MAC Frame (00).

• The C bits indicate what type of management frame the frame is:

• If the E bit is set

to 1, this frame will have a FCS error and must

be retransmitted

by source device.

• If the A bit is set

to 1, the address has been recognized.

• C is set to 1 if the frame has been copied.

• Both A/C bits are duplicated, because the FCS does not cover this field.

up to 17800 bytes

48 48 8 8

8

Field length

in bits unless specified

Figure 4.5 Token Ring frame formats.

Token Ring networks elect two devices called the active monitor andstandby active monitor Any device on the ring can perform this func-tion The function performed by the active monitor is basically to ensurethe integrity of the ring and ensure no device is holding onto the tokenfree frame forever In case a token free frame is lost or corrupted, forexample, the active monitor will clear or purge the ring and issue a newtoken The standby active monitor waits for a failure on the active monitor.

If there’s a token failure (such as a station waits for a token but does not see it for

a specified frame or a faulty device does not release the token or continually sendsdata irrespective of other devices), a special frame advertises that there is no to-

ken available This process is called beaconing When beaconing occurs, the ring

is down, and all stations will re-insert into the ring Beaconing is not a desirableprocess for a network, because it indicates a possible hardware fault During thebeaconing process, a Token Ring station is unable to send data

Let’s now look at how a Token Ring station attaches, or inserts, itself into a ring

using a procedure called the ring insertion process.

Ring Insertion Process

All devices on a Token Ring will go through a process to insert into the ring Thesteps involved in the ring insertion process are as follows:

1 Phase 0: Lobe Media Check/Physical Insertion—The device runs a loopback

test to ensure that any frame that is sent is received

Table 4.4 Function performed by Token Ring stations.

Active Monitor Can be any station on a ring The main function is to provide

timing information and maintenance functions One of the main functions of the Active Monitor is to ensure that frames will not circulate the ring forever A bit in the Token Ring frame called the monitor bit ensures that the frame will only circulate the ring once.

Standby Monitor Can be any station on a ring This station monitors the current

active monitor and replaces it if it becomes available.

Ring Error Monitor Typically a network analyzer This monitor collects errors and

other data seen on the Token Ring.

Beacon A special frame that indicates a problem on the ring A beacon

is sent out by a by a station when a free token frame has not been seen for an amount of time.

Ring Purge A recovery action performed by an active monitor in instances

when a recovery of the ring is required.