CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 39 docx

Because fiber is being used as the transmission medium, the likelihood that an error in the data might occur during the passage of the Ethernet packet across the network is very low, much

to expect that its evolution will cease The higher speeds and greater transmission

dis-tances that are making Ethernet both a LAN and a MAN protocol are not the only

additions to the Ethernet standard that we are likely to see Because fiber is being used

as the transmission medium, the likelihood that an error in the data might occur during

the passage of the Ethernet packet across the network is very low, much lower than in

the original Ethernet On a network with a very low error rate, it makes sense to

trans-mit larger packets of data

The upper limit on the amount of data that can be carried in an Ethernet packet (a

frame) is 1500 bytes Sending more data than that in a frame would make it an invalid

Ethernet frame and cause the network to discard it This has been the standard since

Ethernet was created Given a low likelihood of errors on a network, large files could

be moved over the network more efficiently if a larger amount of data could be carried

in each frame The reason for this is that it takes time for computers to generate and to

process Ethernet headers and trailers Each Ethernet frame must have a header and a

trailer For example, if six times as much data could be sent per frame, there would be

fewer frames (only one sixth as many) needed to carry all the data in a file This means

that fewer headers and trailers would have to be generated by the transmitter and

pro-cessed by the receiver The result is a shorter amount of time needed to move a large

file over a network between two computers WANs that use fiber as their transmission

medium routinely transmit large data packets

For this reason, especially when multigigabit LANs are connected to WANs, it is likely

that we will see the use of Jumbo Ethernet frames A Jumbo frame is any Ethernet frame

that is carrying more than 1500 bytes of data The proposed upper limit for the amount

of data carried in a Jumbo frame is about 9,000 bytes Jumbo frames are not currently

a part of the new IEEE 802.3ae standard However, it is very likely that some vendors

of Ethernet networking equipment will allow Jumbo frames to be carried on Ethernet

networks built using only their equipment This might force the IEEE 802.3 committee

to make support for larger frame sizes an option in new multigigabit standards

Table 6-13 shows the parameters for 10-Gb Ethernet operation

Table 6-13 Parameters for 10-Gbps Ethernet Operation

Parameter Value

continues

Amazingly, 10GbE uses the same frame format (with a few special case exceptions) as 10-, 100-, and 1000-Mbps Ethernet

10GbE Media, Connections, and Architecture

10-Gb Ethernet is a tenfold increase in speed over Gigabit Ethernet Just as with Gigabit Ethernet, with this increase in speed comes extra requirements—the bits being sent get shorter in duration (1 ns), occur more frequently, and require more careful timing In addition, their transmission requires frequencies closer to medium bandwidth limitations and they become more susceptible to noise In response to these issues of synchroniza-tion, bandwidth, and SNR, two separate encoding steps are used by 10-Gb Ethernet The basic idea is to use codes—which can be engineered to have desirable properties—

to represent the user data in a way that is efficient to transmit, including synchronization, efficient usage of bandwidth, and improved SNR characteristics

Bit patterns from the MAC sublayer are converted into symbols, with symbols some-times controlling information (such as start frame, end frame, and medium idle condi-tions) The entire frame is broken up into control symbols and data symbols (data code groups) All of this extra complexity is necessary to achieve the tenfold increase in net-work speed over Gigabit Ethernet 8B/10B encoding (similar to the 4B/5B concept) is used, followed by several different types of line encoding on the optical fiber

* 10-Gbps Ethernet does not permit half-duplex operation, so parameters related to slot timing and collision handling do not apply.

** The value listed is the official interframe spacing.

*** The interframe spacing stretch ratio applies exclusively to 10GBASE-W definitions.

Table 6-13 Parameters for 10-Gbps Ethernet Operation (Continued) Parameter Value

Figure 6-23 represents what happens to the 8B-10B before it is line-encoded 10-Gb

Ethernet uses a variety of complex encodings before line encoding, including 8B/10B

and 64B/66B Bits from these codes then are converted to line signals: low power light

for binary 0 and higher power light for binary 1 Complex serial bit streams are used

for all versions of 10GbE except for 10GBASE-LX4, which uses wide

wavelength-division multiplexing (WWDM) to multiplex 4-bit simultaneous bit streams as four

wavelengths of light launched into the fiber at one time

Figure 6-23 How 10GbE Converts MAC Frames to Four Lanes of Bits

Figure 6-23 shows how 10GbE converts MAC frames to four lanes of bits for parallel

transmission on four wire pairs of UTP or as a bit stream that is then serialized for

laser transmission on single-mode fiber

Figure 6-24 represents the particular case of using four slightly different-colored laser

sources Upon receipt from the medium, the optical signal stream is demultiplexed into

four separate optical signal streams The four optical signal streams then are converted

back into four electronic bit streams as they travel in approximately the reverse

pro-cess back up through the sublayers to the MAC sublayer

Currently, most 10GbE products are in the form of modules (line cards) for addition to

high-end switches and routers As the 10GbE technologies evolve, an increasing

diver-sity of signaling components can be expected As optical technologies involve, improved

transmitters and receivers will be incorporated into these products, further taking

advantage of modularity All 10GbE varieties use optical-fiber media Fiber types

include 10µm single-mode fiber, and 50µm and 62.5 µm multimode fibers A range of

fiber attenuation and dispersion characteristics are supported, but they limit operating

distances

MAC Layer

Reconciliation Sublayer XGMII

Preamble SFD Destination MAC Address

Lane 0 Lane 1 Lane 2 Lane 3 / Start /

000011111010000000010 0

0 0 0 0 0 0000

Figure 6-24 10GBASE-LX4 Signal Multiplexing

SC fiber optic connectors most commonly are used Because optical fiber is the medium used by 10GbE, typically a fiber pair connects Tx for device 1 to Rx for device 2, and vice versa The primary devices connecting currently via 10GbE are high-end modular switches and routers Table 6-14 lists the pinout options for 10GbE

10-Gb Ethernet is available in full-duplex mode only and runs only over optical fiber Hence, collisions are nonexistent and CSMA/CD is unnecessary

As 10GbE standards and products evolve, a wide range of architectures and applica-tion guidelines is becoming possible Most important to consider is that the addiapplica-tion of 10GbE, with its LAN, SAN, MAN, and WAN capabilities, enables network engineers

to consider very sophisticated end-to-end Ethernet networks LAN, SAN, MAN, and WAN topologies using Gigabit Ethernet all are being implemented

10-Gb Ethernet is supported only over fiber-optic media Support is available for 62.5 µm and 50 µm multimode fiber, as well as 10 µm single-mode fiber Even though support is limited to fiber-optic media, some of the maximum cable lengths are surprisingly short

No repeater is defined for 10-Gb Ethernet because half duplex explicitly is not supported

Table 6-14 10GbE Pinout

PMA PMD

Signal Retiming

Optical

WD Multiplexer

Fiber-Optic Medium

As with 10-Mbps, 100,-Mbps and 1000-Mbps versions, it is possible to modify some

of the architecture rules slightly Possible architecture adjustments are related to signal

loss and distortion along the medium Because of dispersion of the signal and other

issues, the light pulse becomes undecipherable beyond certain distances Refer to the

technical timing and spectral requirements in the current 802.3 standard, as well as

the technical information about your hardware performance, before attempting any

adjustments to the architecture rules

Table 6-15 shows the 10-Gb Ethernet implementations Both R and W specifications

are covered by each appropriate entry (for example, 10GBASE-E covers both

10GBASE-ER and 10GBASE-EW)

Note the versatility of 10GbE A diverse set of fiber types and laser sources can be used

to achieve not only LAN, but also MAN and WAN distances

Table 6-15 10-Gb Ethernet Implementations

Implementation Wavelength Medium

Minimum Modal Bandwidth

Operating Distance

*The standard permits 40-km lengths if link attenuation is low enough.

The Future of Ethernet

As the last several sections have documented, Ethernet has gone through an evolution from legacy to Fast to Gigabit to multigigabit technologies Although other LAN tech-nologies are still in place (legacy installations), Ethernet dominates new LAN installa-tions—so much so that some have referred to Ethernet as the LAN “dial tone.” Ethernet

is now the standard for horizontal, vertical, and interbuilding connections In fact, recently developing versions of Ethernet are blurring the distinction between LANs, MANs, and WANs in terms of geographic distance covered as part of one network Figure 6-25 illustrates the expanding scope of Ethernet

Figure 6-25 Ethernet’s Expanding Scope

Although Gigabit Ethernet is now widely available and 10-Gb products becoming more available, the IEEE and the 10-Gb Ethernet Alliance currently have released 40-Gbps, 100-Gbps, and even 160-Gbps standards Which technologies actually are adopted will depend on a number of factors, including the rate of maturation of the technologies and standards, the rate of adoption in the market, and cost

Proposals for Ethernet arbitration schemes other than CSMA/CD have been made But the problem of collisions, so fundamental to physical bus topologies of 10BASE5, 10BASE2, 10BASE-T, and 100BASE-TX hubs, are no longer so common Use of UTP and optical fiber, both of which have separate Tx and Rx paths, and the decreasing costs of switched instead of hubbed connections, make single shared-media, half-duplex media connections much less important

LAN ÒDial ToneÓ

Significant MAN Implementations Special WAN Applications;End-to-End Ethernet, TCP/IP

Networks

The future of networking media is threefold:

■ Wireless (approaching 100 Mbps, perhaps more)

■ Optical fiber (currently at 10,000 Mbps and soon to be more)

Unlike copper and wireless media, in which certain physical and practical limitations

on the highest-frequency signals that can be transmitted are being approached, the

bandwidth limitation on optical fiber is extremely large and is not a limiting factor for

the foreseeable future In fiber systems, the electronics technology (such as emitters

and detectors) and the fiber-manufacturing processes most limit the speed Therefore,

upcoming developments in Ethernet likely will be heavily weighted toward laser light

sources and single-mode optical fiber

When Ethernet was slower, half duplex (subject to collisions and a “democratic” process

for prioritization) was not considered to have the quality of service (QoS) capabilities

required to handle certain types of traffic This included such things as IP telephony

and video multicast

However, the full-duplex, high-speed Ethernet technologies that now dominate the

market are proving to be sufficient at supporting even QoS-intensive applications This

makes the potential applications of Ethernet even wider Ironically, end-to-end QoS

capability helped drive a push for ATM to the desktop and to the WAN in the

mid-1990s, but now Ethernet, not ATM, approaching this goal

At 30 years old, Ethernet technologies continue to grow and have a very bright future

Ethernet Switching

As more nodes are added to an Ethernet physical segment, the contention for the

medium increases The addition of more nodes increases the demands on the available

bandwidth and places additional loads on the medium With the additional traffic, the

probability of collisions increases, resulting in more retransmissions A solution to the

problem is to break the large segment into parts and separate it using Catalyst switches

This isolates these newly segmented sections into isolated collision domains This reduces

the number of collisions and increases the reliability of the network

Bridging and switching are technologies that decrease congestion in LANs by reducing

traffic and increasing bandwidth LAN switches and bridges, operating at Layer 2 of

the OSI reference model, forward frames based on the MAC addresses to perform the

switching function If the Layer 2 MAC address is unknown, the device floods the

frame in an attempt to reach the desired destination LAN switches and bridges also

forward all broadcast frames The result could be storms of traffic being looped end-lessly through the network The Spanning Tree Protocol (STP) is a loop-prevention protocol; it is a technology that enables switches to communicate with each other to discover physical loops in the network

The sections that follow introduce Layer 2 bridging, Layer 2 switching, switching modes, and the Spanning Tree Protocol (STP)

Layer 2 Bridging

A bridge is a Layer 2 device designed to create two or more LAN segments, each of

which is a separate collision domain In other words, bridges were designed to create more usable bandwidth The purpose of a bridge is to filter traffic on a LAN to keep local traffic local, yet allow connectivity to other parts (segments) of the LAN for

traf-fic that is directed there To filter or selectively deliver network traftraf-fic, bridges build tables of all MAC addresses located on a network segment and other networks, and map them to associated ports The process is as follows:

■ If data comes along the network medium, a bridge compares the destination MAC address carried by the data to MAC addresses contained in its tables

■ If the bridge determines that the destination MAC address of the data is from the same network segment as the source, it does not forward the data to other

seg-ments of the network This process is known as filtering By performing this

process, bridges significantly can reduce the amount of traffic between network segments by eliminating unnecessary traffic

■ If the bridge determines that the destination MAC address of the data is not from the same network segment as the source, it forwards the data to the appropriate segment

■ If the destination MAC address is unknown to the bridge, the bridge broadcasts the data to all devices on a network except the one on which it was received The

process is known as flooding.

Generally, a bridge has only two ports and divides a collision domain into two parts All decisions made by a bridge are based on MAC addresses or Layer 2 addressing, and do not affect the logical or Layer 3 addressing Thus, a bridge divides a collision domain but not a logical or broadcast domain No matter how many bridges are in a network, unless a device such as a router works on Layer 3 addressing, all of the net-work will share the same logical (broadcast) address space A bridge will create more (and smaller) collision domains but will not add broadcast domains Because every device on the network must pay attention to broadcasts, bridges always forward them Therefore, all segments in a bridged environment are considered to be in the same broadcast domain

Layer 2 Switching

LAN switches are essentially multiport bridges that use microsegmentation to reduce

the number of collisions in a LAN and increase the bandwidth LAN switches also

support features such as full-duplex communication and multiple simultaneous

con-versations Figure 6-30 shows a LAN with three workstations, a LAN switch, and the

LAN switch’s address table The LAN switch has four interfaces (or network

connec-tions) Stations A and C are connected to the switch’s Interface 3, and Station B is on

Interface 4 As indicated in Figure 6-26, Station A needs to transmit data to Station B

Figure 6-26 LAN Switch Operation

Remember that as this traffic goes through the network, the switch operates at Layer 2,

meaning that the switch can look at the MAC layer address When Station A transmits

and the switch receives the frames, the switch assesses the traffic as it goes through to

dis-cover the source MAC address and store it in the address table, as shown in Figure 6-27

Figure 6-27 Building an Address Table

B

1 2 3

4

10 Mbps

10 Mbps

Interface

Data from A to B

B

1 2 3

4

10 Mbps

10 Mbps

Interface

As the traffic goes through the switch, an entry is made in the address table identifying the source station and the interface that it is connected to on the switch The switch now knows where Station A is connected When that frame of data is in the switch, it floods to all ports because the destination station is unknown, as shown in Figure 6-28

Figure 6-28 Flooding Data to All Switch Ports

After the address entry is made in the table, however, a response comes back from Station B to Station A The switch now knows that Station B is connected to Inter-face 4, as shown in Figure 6-29

Figure 6-29 Responding to the Flooding Message

The data is transmitted into the switch, but notice that the switch does not flood the traffic this time The switch sends the data out of only Interface 3 because it knows where Station A is on the network, as shown in Figure 6-30

B

1 2 3 4

10 Mbps Interface

10 Mbps

Data from A to B

B

1 2 3 4

10 Mbps Interface

10 Mbps

X B