Tài liệu học CCNA kỳ 3 ccna3 mod4 switchingconcepts

Học viện mạng Cisco Bách Khoa - Website: www.ciscobachkhoa.com 3Ethernet/802.3 LAN development The earliest LAN technologies commonly used either thick Ethernet or thin Ethernet infrast

Trang 1

Module 4 Switching Concepts

CCNA 3 version 3.1

Contents

• Introduction to Ethernet/802.3 LANs

• Introduction to LAN Switching

• Switch Operation

Trang 2

Học viện mạng Cisco Bách Khoa - Website: www.ciscobachkhoa.com 3

Ethernet/802.3 LAN development

The earliest LAN technologies commonly used either thick

Ethernet or thin Ethernet infrastructures

Ethernet/802.3 LAN development ( cont…)

• A hub is a Layer 1 device

• Sometimes referred to as an Ethernet concentrator or a multi-port repeater

• Allowed greater access to the network for more users

• Active hubs also allowed for the extension of networks to greater distances

• A hub does not make any decisions when receiving data signals It simply regenerates and

amplifies the data signals

Trang 3

• Layer 2 devices are more intelligent than Layer 1 devices

• Layer 2 devices make forwarding decisions based on Media Access Control (MAC)

• A bridge is a Layer 2 device.

• Bridge collect and pass frames between two network segments.

• Bridges control traffic to the network.

• Bridges do not restrict broadcast traffic

• A switch is also a Layer 2 device and may be referred to as a multi-port bridge

• Make forwarding decisions based on MAC addresses

• Create a virtual circuit between two connected devices that want to

communicate

• Also able to facilitate multiple, simultaneous virtual circuit connections.

Trang 4

• A router is a Layer 3 device

• Makes decisions based on groups of network addresses , or classes.

• Use routing tables to record the Layer 3 addresses of the networks.

network

• LANs typically employ a combination of Layer 1, Layer 2, and Layer 3

devices.

• Implementation of these devices depends on factors that are specific to

the particular needs of the organization

Trang 5

Factors that impact network performance

• Too many users on a 10-Mbps segment

• Most users accessing one or two servers

• Network-intensive applications such as color publishing,

CAD/CAM, imaging and relational databases

Elements of Ethernet/802.3 networks

• Ethernet is used to transport data between

devices on a network

• These devices include computers, printers, and

file servers…

Trang 6

Elements of Ethernet/802.3 networks (cont…)

The performance of a shared medium Ethernet/802.3 LAN can be

negatively affected by several factors:

• The data frame delivery of Ethernet/802.3 LANs is of a broadcast

nature

• The carrier sense multiple access/collision detect (CSMA/CD)

method allows only one station to transmit at a time

• Multimedia applications with higher bandwidth demand such as

video and the Internet, coupled with the broadcast nature of

Ethernet, can create network congestion

• Normal latency occurs as the frames travel across the Layer 1

medium and through Layer 1, Layer 2, and Layer 3 networking

devices

• Extending the distances and increasing latency of the

Ethernet/802.3 LANs by using Layer 1 repeaters.

• Ethernet using CSMA/CD and a shared medium can support data

transmission rates of up to 100 Mbps

• CSMA/CD is an access method that allows only one station to transmit at

a time

Trang 7

A certain number of collisions are expected in the design of

Ethernet and CSMA/CD Æ collisions can become a major

problem in a CSMA/CD network

Network congestion

• The requirements have exceeded the 10 Mbps available on shared Ethernet/802.3 LANs

• Today's networks are experiencing an increase in the transmission of many forms of media:

– Large graphics files

– Images – Full-motion video – Multimedia applications

Trang 8

Network latency

Latency, or delay, is the time a frame or a packet takes to travel from the

source station to the final destination

Latency has at least three sources:

• The time source NIC place voltage pulses on the wire and the time the

receiving NIC interpret these pulses

• The actual propagation delay as the signal takes time to travel along the cable

• Latency is added according to which networking devices.

Ethernet 10 BASE-T transmission time

Transmission time equals the number of bits being sent

times the bit time for a given technology

Another way to think about transmission time is the time it

takes a frame to be transmitted

Trang 9

The benefits of using repeaters

• Allow a longer end-to-end distance

• Increase the collision domain size

• Increase the broadcast domain size

Full-duplex transmitting

• Allows the transmission of a packet and the reception of a

different packet at the same time

• Requires the use of two pairs of wires

• Collision-free transmission

• Offers 100% of the bandwidth in both directions

Trang 10

LAN segmentation

• A network can be divided into smaller units called segments

• Why segment LANs ?

– Isolates traffic between segments

– Achieves more bandwidth per user by creating smaller collision

domains

LAN segmentation ( cont … )

• Each segment uses the CSMA/CD access method and maintains traffic

between users on the segment.

• Each segment is its own collision domain

• Segmentation allows network congestion to be significantly reduced

• Devices within a segment share the total available bandwidth

Trang 11

LAN segmentation with bridges

• Bridges read the sender's MAC address of the data packets to discover

which devices are on each segment

• The MAC addresses are then used to build a bridging table Æ block

packets that do not need to be forwarded from the local segment

LAN segmentation with routers

• More manageable, greater functionality, multiple active paths

• Smaller broadcast domains

• Operates at Layer 3

• Adding a latency factor of 20% to 30% over a switched network

• Not forward broadcasts

Trang 12

LAN segmentation with switches

• A switch eliminates the impact of collisions through microsegmentation

• Low latency and high frame-forwading rates at each interface port

• Works with existing 802.3 (CSMA/CD) compliant network interface cards

and cabling

• All hosts connected to the switch are still in the same broadcast domain

LAN segmentation with switches (cont…)

Virtual network circuit is established within the switch and

exists only when the nodes need to communicate Æ

maximum available bandwidth

Trang 13

Basic operations of a switch

Two basic operations:

– Switching data frames

– Maintaining switching operations

Basic operations of a switch (cont…)

Trang 14

Trang 15

Ethernet switch latency

Latency is the period of time from when the beginning of a frame

enters to when the end of the frame exits the switch

• Directly related to the configured switching process and volume of

traffic

• Measured in fractions of a second

Trang 16

Layer 2 switching

Layer 3 switching

Trang 17

Symmetric switching

A symmetric switch provides switched connections

between ports with the same bandwidth

Asymmetric switching

An asymmetric LAN switch provides switched connections between

ports of unlike bandwidth

• Requires the switch to use memory buffering

Trang 18

Memory buffering

• In port-based memory buffering frames are stored in

queues that are linked to specific incoming ports

• Shared memory buffering deposits all frames into a

common memory buffer which all the ports on the switch

share

Switching methods

• Store-and-forward – The entire frame is received before

any forwarding takes place

• Cut-through – The frame is forwarded through the switch

before the entire frame is received

Trang 19

Switching methods (cont…)

The following are two forms of cut-through

switching:

forwards a packet after reading the

destination address

filters out collision fragments ( < 64 bytes )

before forwarding begins

Frame transmission modes

• Fast-forward - immediately forwards a packet after reading the destination

address

• Store-and-forward – the entire frame is received before any forwarding takes

place

• Fragment-free – reads the first 64 bytes of an Ethernet frame and then begins

forwarding it to the appropriate port or ports

Trang 20

Frame transmission modes (cont…)

Adaptive cut-through :

• A combination of cut-through and store-and-forward

• Uses cut-through until it detects a given number of errors

• Once the error threshold is reached, the switch changes to

store-and-forward mode

How switches and bridges learn addresses

• Bridges and switches learn in the following ways:

– Reading the source MAC address of each received frame or datagram

– Recording the port on which the MAC address was received.

• The bridge or switch learns which addresses belong to the devices connected to each port

• The learned addresses and associated port or interface are stored in the addressing table

• The bridge examines the destination address of all received frames

• The bridge then scans the address table searching for the destination address

Trang 21

How switches and bridges filter frames

• Switches flood frames that are:

– Layer 2 broadcasts

– Multicasts (unless running multicast snooping or IGMP)

– Multicast are special layer 2 and layer 3 addresses that are sent to devices

that belong to that “group”

Microsegmentation implementation

A switch forms dedicated paths between sending and receiving hosts

Trang 22

Switches and collision domains

• The switch does this by creating dedicated network segments, or point-to-point connections.

Collision Domains

Switches and broadcast domains

• When a device wants to send out a Layer 2 broadcast, the destination

MAC address in the frame is set to all ones

• A MAC address of all ones is FF:FF:FF:FF:FF:FF in hexadecimal

• By setting the destination to this value, all the devices will accept and

process the broadcasted frame.

Trang 23

Communication between switches and workstation

Summary

• The earliest LAN technologies commonly used either thick Ethernet or thin

Ethernet infrastructures.

• The most common LAN architecture is Ethernet Ethernet is used to transport

data between devices on a network These devices include computers, printers,

and file servers.

• A network can be devided into smaller units called segments.

• Segmentation allows network congestion to be significantly reduced within each

segment.

• LAN switching decreases bandwidth shortages and network bottlenecks, such

as those between several workstations and a remote file server.

• The two form switching modes are: store and forward and cut through.

• The two forms of cut-through switching are: fast forward and fragment fee.

• A switch is a network device that selects a path or circuit for sending a frame to

its destination Both switches and bridges operate at Layer 2 of the OSI model.

Định dạng
Số trang	23
Dung lượng	801,3 KB