CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 40 pptx

Figure 6-31 Switching Technology: Full Duplex Latency Latency , sometimes called propagation delay, is the time that a frame, or packet, of data takes to travel from the source station

Figure 6-30 Transmitting Data to a Known Station

The original transmission indicated where that MAC address came from, enabling the

switch to deliver traffic in the network more efficiently

Microsegmentation

As technology improved, it became possible to build bridges with more capability

Naturally, the ultimate goal is to have only one node on each port of a bridge This

would reduce the collision domains so that collisions would be nearly nonexistent A

switch does exactly that and is simply a bridge with many ports These small physical

segments are called microsegments.

Microsegmentation facilitates the creation of a dedicated segment and provides

dedi-cated bandwidth to each user on the network Each user receives instant access to the

full bandwidth and does not have to contend for available bandwidth with other users

This means that pairs of devices on the same switch can communicate in parallel with

a minimum number of collisions Microsegmentation reduces collisions in a network

and effectively increases the capacity for each station connected to the network

In addition to faster microprocessors and memory, two other technological advances

made switches possible Content Addressable Memory (CAM) is memory that essentially

works backward compared to conventional memory Entering data into the memory

returns the associated address Using CAM allows a switch to directly find the port

that is associated with a MAC address without using search algorithms An

application-specific integrated circuit (ASIC) is a device consisting of undedicated logic gates that

can be programmed to perform functions at logic speeds Operations that might have

been done in software now can be done in hardware using an ASIC The use of these

technologies greatly reduced the delays caused by software processing and enabled a

switch to keep pace with the data demands of many microsegments and high bit rates

B

1 2 3 4

10 Mbps Interface

10 Mbps

Data from B to A

X B

Full-Duplex Transmission

Another function of LAN switching that dramatically improves bandwidth is full-duplex transmission, which effectively doubles the amount of bandwidth between nodes Full-duplex transmission between stations is achieved by using point-to-point Ethernet connections This feature can be important, for example, between high-band-width consumers, such as a connection between a switch and a server Full-duplex transmission provides a collision-free transmission environment Because both nodes can transmit and receive at the same time, there are no negotiations for bandwidth

In 10-Mbps connections, for example, full-duplex transmission provides 10 Mb of transmit capacity and 10 Mb of receive capacity, for effectively 20 Mb of capacity on

a single connection Likewise, a 100-Mbps connection offers effectively 200 Mbps of throughput, as illustrated in Figure 6-31 Full-duplex communication also supports two data transmission paths, with speeds up to 1 Gbps

Figure 6-31 Switching Technology: Full Duplex

Latency

Latency , sometimes called propagation delay, is the time that a frame, or packet, of

data takes to travel from the source station or node to its final destination on the net-work A wide variety of conditions can cause delays as a frame travels from source to destination:

■ Media delays caused by the finite speed that signals can travel through the physi-cal media

■ Circuit delays caused by the electronics that process the signal along the path

switching and protocols

■ Delays caused by the content of the frame and where in the frame switching deci-sions can be made For example, a device cannot route a frame to a destination until the destination MAC address has been read

Latency is the time delay between when a frame first starts to leave the source device and when the first part of the frame reaches its destination

10 Mbps, 100 Mbps, or 1 Gbps 10 Mbps, 100 Mbps, or 1 Gbps

10 Mbps, 100 Mbps, or 1 Gbps 10 Mbps, 100 Mbps, or 1 Gbps

Full Duplex

Switch Modes

How the content of a frame is switched to the destination port is a trade-off of latency

and reliability The three modes of switching—store-and-forward, cut-through, and

fragment-free switching—offer different performance and latency

Store-and-Forward Switching

Instore-and-forward switching, the switch reads the entire frame of data, checks the

frame for errors, decides where it needs to go, and then sends it on its way Figure 6-32

illustrates the operation of store-and-forward switching The obvious trade-off here

is that it takes the switch longer to read the entire frame As it reads the entire frame,

however, it detects any errors on that frame If the frame is in error, the frame is not

forwarded and is discarded Although cut-through switching is faster, it offers no error

detection The latency introduced by store-and-forward switching is usually not a

sig-nificant issue

Figure 6-32 Store-and-Forward Switching

Cut-Through Switching

Incut-through switching, the switch reads the beginning of the frame up to the

destina-tion MAC address as the traffic flows through the switch and “cuts through” to its

des-tination without continuing to read the rest of the frame, as illustrated in Figure 6-33

Cut-through switching decreases the latency of the transmission However, cut-through

switching has no error detection

Checked the frame It is good I am to forward the frame now.

Figure 6-33 Cut-Through Switching

Fragment-Free Switching

Fragment-free switching is a modified form of cut-through switching Fragment-free switching filters out collision fragments, which are the majority of packet errors, before forwarding begins Fragment-free switching waits until the received packet has been determined not to be a collision fragment before forwarding the packet

When to Use Each Switching Mode

When using cut-through and fragment-free modes, both the source port and the desti-nation port must be operating at the same bit rate to keep the frame intact This is

called synchronous switching If the bit rates are not the same, the frame must be stored

at one bit rate before it is sent out at the other bit rate This is known as asynchronous

switching Store-and-forward mode must be used for asynchronous switching

Asym-metric switching provides switched connections between ports of unlike bandwidths, such as a combination of 100 Mbps and 1000 Mbps Asymmetric switching is opti-mized for client/server traffic flows in which multiple clients simultaneously communi-cate with a server, requiring more bandwidth dedicommuni-cated to the server port to prevent a bottleneck at that port

Introduction to the Spanning Tree Protocol

When multiple switches are connected, there is a possibility of creating a loop where there is no clear path from source to destination If switches are arranged in a simple hierarchical tree, no loops will occur, as shown in Figure 6-34

I have the destination MAC address I can transmit now.

Figure 6-34 STP Reducing Routing Loops

However, when extra switches and bridges are added to provide redundant paths for

reliability and fault tolerance, loops can occur, as shown in Figure 6-35

Figure 6-35 Broadcast Storms

Hub Cat-6

Cat-7

Cat-1

Cat-3

Cat-2

Cat-4

Cat-5

Wiring Closet Backbone

Server Farm

Ethernet

Ethernet Host 1

In Figure 6-35, the following steps are occurring:

resulting in routing loops

To counteract the possibility of loops, switches are provided with a protocol for them

to talk with each other to resolve the condition A switch sends special messages called

bridge protocol data units (BPDUs) out all its ports to let other switches know of its existence, as shown in Figures 6-36 and 6-37 The switches use a spanning tree algo-rithm (STA) to resolve and shut down the redundant paths The process of shutting

down a port is called blocking The result of resolving and eliminating the loops is a

logical hierarchical tree created with no loops However, the alternate paths are still available, in case they are needed The protocol used to resolve and eliminate loops is known as the Spanning Tree Protocol (STP) This creates another switch and bridge operation mode known as loop-avoidance mode

Figure 6-36 BPDU Communication

LAN Switch 1

Bridge ID 12345111 Port 2

LAN Switch 3

Bridge ID 12345556 Port 1

AA-11

Switch 1’s Bridge ID is

lower and wins the comparison.

BPDU

Priority: 1 Root: 12345555 Path Cost: 0 Bridge ID: 12345555 Port ID: 8001

BPDU

Priority: 1 Root: 12345111 Path Cost: 0 Bridge ID: 12345111 Port ID: 8002

100 Mbps Fast Ethernet Segment All Other Segments Are

10 Mbps

Figure 6-37 BPDU Protocol Layout

Consequently, switches have five operating modes:

■ Blocking—A port in blocking state sends and listens to BPDUs but does not

for-ward frames By default, all ports are in blocking state when the switch is turned on

■ Listening—In listening state, a port listens to the BPDUs to make sure there are

no loops on the network No frames are forwarded in this state

■ Learning—In this state, a port learns MAC addresses and builds a address table,

but it does not forward frames

■ Forwarding—A port in the forwarding state forwards frames BPDUs are sent

and listened to

■ Disabled—A port in the disabled state does not participate in the operation of

STP Therefore, it does not listen to BPDUs or forward frames

Figure 6-38 illustrates some of the port states and operating modes in a switched

net-work using STP

Lab Activity Introduction to Fluke Network Inspector

This lab is a tutorial demonstrating how to use the Fluke Networks Network Inspector (NI) to discover and analyze network devices within a broadcast domain This lab demonstrates the key features of the tool that can be incorpo-rated into various troubleshooting efforts in the remaining labs

Root BID Root Path Cost Sender BID Port ID

Who is the root bridge?

How far away is the root bridge?

What is the BID of the bridge that sent this BPDU?

What port on the sending bridge did this BPDU come from?

Figure 6-38 Port States

Summary

In this chapter, you learned the following key points:

■ Several types of Ethernet exist: Ethernet, Fast Ethernet, Gigabit Ethernet, and 10-Gb Ethernet Each type is associated with a different transfer rate

■ Ethernet uses carrier sense multiple access collision detect (CSMA/CD)

■ 10-Mbps Ethernet operates within the timing limits offered by a series of no more than five segments separated by no more than four repeaters

number of collisions to a minimum and increases the effective bandwidth

address of the frame packet

Lab Activity Introduction to Fluke Protocol Inspector

This lab is a tutorial demonstrating how to use the Fluke Networks Protocol Inspector to analyze network traffic and data frames This lab demonstrates key features of the tool that can be incorporated into various troubleshooting efforts in the remaining labs

Segment 1 Forwarding 1/1

Designated Port

Root Port Forwarding 1/1

1/2 Forwarding DesignatedPort

Cat-A

Cat-B

Segment 3

1/2

1/1

1/2

Root Port Forwarding

Forwarding Designated Port

Segment 2

Non-Designated Port Blocking Root Bridge

Cat-C

■ Full-duplex communication allows two devices to communicate with each other

simultaneously and effectively doubles the throughput that the LAN switch can translate

store-and-forward, cut-through, and fragment-free switching

of network loops on a Layer 2 network

■ The ports on a bridge or switch using STP exist in one of the following five

states: blocking, listening, learning, forwarding, or disabled

To supplement all that you’ve learned in this chapter, refer to the chapter-specific Videos,

PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book

Key Terms

10BASE2 10-Mbps baseband Ethernet specification using 50-ohm thin coaxial cable 10BASE2, which is part of the IEEE 802.3 specification, has a distance limit of 185m (606 ft.) per segment

10BASE5 10-Mbps baseband Ethernet specification using standard (thick) 50-ohm baseband coaxial cable 10BASE5, which is part of the IEEE 802.3 baseband physical layer specification, has a distance limit of 500m (1640 ft.) per segment

10BASE-T 10-Mbps baseband Ethernet specification using two pairs of twisted-pair cabling (Category 3, 4, or 5): one pair for transmitting data and the other for receiving data 10BASE-T, which is part of the IEEE 802.3 specification, has a distance limit of approximately 100m (328 ft.) per segment

100BASE-FX 100-Mbps baseband Fast Ethernet specification using two strands of multimode fiber-optic cable per link To guarantee proper signal timing, a

100BASE-FX link cannot exceed 400m (1312 ft.) in length It is based on the IEEE 802.3 standard

100BASE-TX 100-Mbps baseband Fast Ethernet specification using two pairs of either UTP or STP wiring The first pair of wires is used to receive data; the second is used to transmit To guarantee proper signal timing, a 100BASE-TX segment cannot exceed 100m (328 ft.) in length It is based on the IEEE 802.3 standard

1000BASE-T 1000-Mbps baseband Gigabit Ethernet specification using four pairs

of Category 5 UTP cable for a maximum length of 100m (328 ft.)

1000BASE-SX 1000-Mbps baseband Gigabit Ethernet specification using a short laser wavelength on multimode fiber-optic cable for a maximum length of 550m (1804.5 ft.)

1000BASE-LX 1000-Mbps baseband Gigabit Ethernet specification using a long wave-length for a long-haul fiber-optic cable for a maximum wave-length of 10,000 (32808.4 ft.)

4D-PAM5 The symbol-encoding method used in 1000BASE-T The four-dimensional quinary symbols (4D) received from the 8B1Q4 data encoding are transmitted using five voltage levels (PAM5) Four symbols are transmitted in parallel each symbol period

8B1Q4 For IEEE802.3, the data-encoding technique used by 1000BASE-T when converting GMII data (8B-8 bits) to four quinary symbols (Q4) that are transmitted during one clock (1Q4)

BPDU (bridge protocol data unit) Spanning Tree Protocol hello packet that is sent out at configurable intervals to exchange information among bridges in the network