Institute of Technology,Sligo Dept of Computing

LAN SwitchingLAN Switching

Semester 3, Chapter 2Semester 3, Chapter 2

Table of ContentsTable of Contents

LAN Communication ProblemsLAN Communication Problems

• Full-Duplex, Fast Ethernet, and Segmentation

• Switching and VLANs

• The Spanning-Tree Protocol

Institute of Technology,Sligo Dept of Computing

LAN Communication LAN Communication ProblemsProblems

Network PerformanceNetwork Performance Network congestion has increased significantly Network congestion has increased significantly

since the mid-90s due to:since the mid-90s due to: Multitasking Operating SystemsMultitasking Operating Systems

multiple simultaneous network transactions (e.g., ftp multiple simultaneous network transactions (e.g., ftp download & browsing)download & browsing)

Faster Processing PowerFaster Processing Power 1980s: 1 MIPS; Today: over 75 MIPS1980s: 1 MIPS; Today: over 75 MIPS

Network-intensive ApplicationsNetwork-intensive Applications accessing network servers to use applications, files, etc.accessing network servers to use applications, files, etc.

Elements of Ethernet/802.3Elements of Ethernet/802.3 CharacteristicsCharacteristics

Most common LAN architectureMost common LAN architecture Used to transport data between devices connected to Used to transport data between devices connected to

the same delivery mediumthe same delivery medium Uses a Uses a data frame broadcast methoddata frame broadcast method

Negative effects of a shared LANNegative effects of a shared LAN broadcast delivery of broadcast delivery of allall frames frames CSMA/CD: collisions are inherentCSMA/CD: collisions are inherent distance limitation requires using repeaters to extenddistance limitation requires using repeaters to extend

Half-Duplex EthernetHalf-Duplex Ethernet PropertiesProperties

Only one host can transmit at a time because Only one host can transmit at a time because the NIC needs to listen for collisionsthe NIC needs to listen for collisions

The NIC provides several circuits. Most The NIC provides several circuits. Most important are:important are:

receive (RX), transmit (TX), and collision detectionreceive (RX), transmit (TX), and collision detection bandwidth usage = 50% to 60%bandwidth usage = 50% to 60%

CSMA/CDCSMA/CD OperationOperation Devices on shared media listen for a carrier before Devices on shared media listen for a carrier before

transmittingtransmitting If no carrier is sensed for a specific period of time, a device If no carrier is sensed for a specific period of time, a device

can transmitcan transmit If two devices transmit simultaneously, a collision occurs. If two devices transmit simultaneously, a collision occurs.

The NIC senses this because it is transmitting and The NIC senses this because it is transmitting and receiving at the same timereceiving at the same time

The first device to detect the collision will The first device to detect the collision will generate a jam generate a jam signalsignal (colliding devices continue to transmit so that all (colliding devices continue to transmit so that all devices will hear the collision)devices will hear the collision)

All devices calculate a backoff algorithm which will delay All devices calculate a backoff algorithm which will delay transmission for a random length of time.transmission for a random length of time.

First device who’s delay time expires can attempt to First device who’s delay time expires can attempt to transmit data.transmit data.

Network CongestionNetwork Congestion Occurs as more people utilize a network to...Occurs as more people utilize a network to...

Share large files (e.g. databases, applications, etc.)Share large files (e.g. databases, applications, etc.) Access file serversAccess file servers connect to the Internetconnect to the Internet

Relieving congestion requiresRelieving congestion requires Increasing the amount of bandwidthIncreasing the amount of bandwidth and/or and/or Using available bandwidth more efficientlyUsing available bandwidth more efficiently

Network LatencyNetwork Latency Latency explainedLatency explained

Represents the time it takes a frame to travel from is Represents the time it takes a frame to travel from is source device to its final destination on the networksource device to its final destination on the network (also (also know as know as propagation delaypropagation delay))

Latency can also be described as the delay between the Latency can also be described as the delay between the time a device requests access to a network and the time time a device requests access to a network and the time it is granted permission to transmitit is granted permission to transmit

For switches and routers, latency is the amount of delay For switches and routers, latency is the amount of delay between the time when the device receives the frame on between the time when the device receives the frame on one interface and forwards that frame out another one interface and forwards that frame out another interfaceinterface

Routers have more inherent latency than a switch. Why?Routers have more inherent latency than a switch. Why?

Ethernet Transmission TimeEthernet Transmission Time DefinedDefined

Transmission time is Transmission time is the time necessary to move a packet from the time necessary to move a packet from the data link layer to the physical layerthe data link layer to the physical layer

10BaseT Transmission Time10BaseT Transmission Time Each Each bitbit has a 100ns window for transmission has a 100ns window for transmission

ns-nanosecond (1 billionth of a second)ns-nanosecond (1 billionth of a second) So each So each bytebyte has what size window? has what size window? A 64 byte frame (the smallest allowed frame) requires 51,200 ns A 64 byte frame (the smallest allowed frame) requires 51,200 ns

or 51.2 microsecondsor 51.2 microseconds Just to frame a 1000 byte packet requires 800 microsecondsJust to frame a 1000 byte packet requires 800 microseconds Additional latency will be added propagating the frame down the Additional latency will be added propagating the frame down the

wire and by any additional devices the frame has to go through wire and by any additional devices the frame has to go through before reaching the destinationbefore reaching the destination

Using RepeatersUsing Repeaters What is attenuation?What is attenuation?

Loss of signal strength as it travels through the Loss of signal strength as it travels through the network; caused by resistance inherent in the mediumnetwork; caused by resistance inherent in the medium

Benefits of Using a RepeaterBenefits of Using a Repeater a layer 1 device that cleans up and boosts the signala layer 1 device that cleans up and boosts the signal extends the coverage area of a LAN segmentextends the coverage area of a LAN segment

Negative Effects of Using a RepeaterNegative Effects of Using a Repeater increases the collision domain sizeincreases the collision domain size increases the broadcast domain sizeincreases the broadcast domain size can’t filter traffic based on Layer 2 or 3 addressingcan’t filter traffic based on Layer 2 or 3 addressing

Institute of Technology,Sligo Dept of Computing

Full-Duplex, Full-Duplex, Fast Ethernet,Fast Ethernet,

and Segmentationand Segmentation

Full-Duplex EthernetFull-Duplex Ethernet Simultaneous TX and RXSimultaneous TX and RX

allows the transmission of a packet and the reception of a allows the transmission of a packet and the reception of a different packet at the same time.different packet at the same time.

requires the use of two pairs of wiresrequires the use of two pairs of wires in the cable and a in the cable and a switched connection between each node.switched connection between each node.

this connection is considered point-to-point and is this connection is considered point-to-point and is collision free.collision free.

because both nodes can transmit and receive at the because both nodes can transmit and receive at the same time, there are no negotiations for bandwidth.same time, there are no negotiations for bandwidth.

100% of bandwidth is available100% of bandwidth is available: 10 Mbps increases to 20 : 10 Mbps increases to 20 Mbps of potential throughput (10 Mbps TX & 10 Mbps Mbps of potential throughput (10 Mbps TX & 10 Mbps RX)RX)

LAN SegmentationLAN Segmentation Benefits of Segmenting the NetworkBenefits of Segmenting the Network

By segmenting a LAN fewer devices are sharing the same bandwidth, improving performance of a shared media LANEach segment is considered its own collision domainHow many broadcast domains in graphic?

Segmenting with BridgesSegmenting with Bridges Bridge OperationBridge Operation

Bridges “learn” a network’s segmentation by Bridges “learn” a network’s segmentation by building address tables that contain:building address tables that contain:

Bridge interface that will reach that deviceBridge interface that will reach that device Each device’s MAC addressEach device’s MAC address

Segmenting with BridgesSegmenting with Bridges Generic Frame FormatGeneric Frame Format

Frame can be any length depending on Frame can be any length depending on technologytechnology

Ethernet frame can be up to 1522 bytes longEthernet frame can be up to 1522 bytes long Address section is 12 bytes (6 bytes for each Address section is 12 bytes (6 bytes for each

MAC)MAC) FCS contain the CRC to check frame for FCS contain the CRC to check frame for

errorserrors

AddressStartFrame

Type/Length

Data FCS StopFrame

Segmenting with BridgesSegmenting with Bridges Bridge PerformanceBridge Performance

adds 10% to 30% latency due to decision-making adds 10% to 30% latency due to decision-making processprocess

considered a considered a store-and-forwardstore-and-forward device because it must device because it must calculate the CRC at the end of the frame to check it calculate the CRC at the end of the frame to check it for errors before forwardingfor errors before forwarding

if the bridge does not have an entry for the destination if the bridge does not have an entry for the destination MAC, it...MAC, it...

adds the source MAC to its bridging tableadds the source MAC to its bridging table forwards the frame out all interfaces except the one it was forwards the frame out all interfaces except the one it was

received onreceived on when a reply returns, it adds the destination MAC to the tablewhen a reply returns, it adds the destination MAC to the table

Segmenting with RoutersSegmenting with Routers Router OperationRouter Operation

Routers...Routers... use layer 3 addressinguse layer 3 addressing (IP, IPX) and routing protocols (RIP, (IP, IPX) and routing protocols (RIP,

IGRP) to determine the path andIGRP) to determine the path and switch the packet out the correct interface to the destinationswitch the packet out the correct interface to the destination

because a router must open the packet to read Layer because a router must open the packet to read Layer 3 addressing, it adds latency3 addressing, it adds latency

In addition, protocols like TCP which require In addition, protocols like TCP which require acknowledgments of every packet can increase acknowledgments of every packet can increase latency, reducing throughput from 20% to 40%latency, reducing throughput from 20% to 40%

Segmenting with RoutersSegmenting with Routers Router BenefitsRouter Benefits

Like switches, Like switches, routers segment routers segment collision domains.collision domains.

However, since a However, since a router will not router will not forward broadcasts, forward broadcasts, it also segments it also segments broadcast domains.broadcast domains.

Each router interface Each router interface represents its own represents its own broadcast domain.broadcast domain.

Segmenting with SwitchesSegmenting with Switches Switching BenefitsSwitching Benefits

a switch is simply a multi-port bridge, a switch is simply a multi-port bridge, making making forwarding decisions based on MAC addressesforwarding decisions based on MAC addresses

so, like a bridge, segmenting a LAN with a switch so, like a bridge, segmenting a LAN with a switch creates more collision domainscreates more collision domains

replacing hubs with switches therefore decreases replacing hubs with switches therefore decreases congestion and increases available bandwidth.congestion and increases available bandwidth.

a switch can a switch can microsegmentmicrosegment a LAN creating collision- a LAN creating collision-free domains but still be in the same broadcast domain.free domains but still be in the same broadcast domain.

switch switch creates a virtual circuits, allowing many users to creates a virtual circuits, allowing many users to communicate in parallelcommunicate in parallel. .

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Switching and VLANsSwitching and VLANs

Switch OperationSwitch Operation Switches perform two basic functions:Switches perform two basic functions:

Building and maintaining switching tables (similar to a Building and maintaining switching tables (similar to a bridge table) based on MAC addressesbridge table) based on MAC addresses

Switching frames out the interface to the destinationSwitching frames out the interface to the destination Differences between switches & bridgesDifferences between switches & bridges

Switches operate at higher speedsSwitches operate at higher speeds Switches are capable of creating virtual LANs Switches are capable of creating virtual LANs

(VLANs) through microsegmentation(VLANs) through microsegmentation Bridges switch using software; switches typically Bridges switch using software; switches typically

switch using hardware (called the “switch fabric”)switch using hardware (called the “switch fabric”)

Switch Latency (Delay)Switch Latency (Delay) A switch adds 21 microseconds of latency. A switch adds 21 microseconds of latency. This can be reduced by using a different This can be reduced by using a different

switching methodswitching method As opposed to As opposed to store-and-forwardstore-and-forward, the switch can , the switch can

use use cut-throughcut-through switching which switches the switching which switches the packet as soon as the destination MAC is read.packet as soon as the destination MAC is read.

How a LAN Switch Learns How a LAN Switch Learns AddressesAddresses

MAC addresses are learned dynamically and are MAC addresses are learned dynamically and are stored in CAM (content-addressable memory)stored in CAM (content-addressable memory)

Each time a switch stores an address entry in the Each time a switch stores an address entry in the table, it is time-stampedtable, it is time-stamped..

The time-stamp is updated each time a frame is The time-stamp is updated each time a frame is receivedreceived

Addresses whose time-stamp expires are deleted Addresses whose time-stamp expires are deleted from the tablefrom the table

This keeps switching tables smallThis keeps switching tables small

Benefits of LAN SwitchingBenefits of LAN Switching Cost-effective; Cost-effective;

switches only cost 3 switches only cost 3 to 5 times that of a to 5 times that of a hubhub

Allows the creation of Allows the creation of virtual circuitsvirtual circuits

More flexibility in More flexibility in managing the networkmanaging the network

Reduces number of Reduces number of collisionscollisions

Works with existing Works with existing 802.3 cabling802.3 cabling

Symmetric SwitchingSymmetric Switching

symmetric switching symmetric switching provides switched provides switched connections between connections between ports with the same ports with the same bandwidthbandwidth (10/10 (10/10 Mbps or 100/100 Mbps or 100/100 Mbps)Mbps)

can cause can cause bottlenecks as users bottlenecks as users try to access servers try to access servers on other segments.on other segments.

potential bottlenecks

Asymmetric SwitchingAsymmetric Switching

asymmetric switching asymmetric switching reduces the likelihood reduces the likelihood of a potential of a potential bottleneck at the bottleneck at the server by attaching server by attaching the segment with the the segment with the server to a higher server to a higher bandwidth port (100 bandwidth port (100 Mbps)Mbps)

asymmetric switching asymmetric switching requires memory requires memory buffering in the switchbuffering in the switch

Memory BufferingMemory Buffering DefinedDefined

Area of memory in a switch where destination and Area of memory in a switch where destination and transmission data are storedtransmission data are stored until it can be switched out the until it can be switched out the correct port.correct port.

Two typesTwo types Port-based memory bufferingPort-based memory buffering

packets are stored in a queue on each portpackets are stored in a queue on each port possible for one packet to delay transmission of other packets possible for one packet to delay transmission of other packets

because of a busy destination portbecause of a busy destination port Shared memory bufferingShared memory buffering

common memory buffering shared by all portscommon memory buffering shared by all ports allows packets to be RX on one port and TX out another port without allows packets to be RX on one port and TX out another port without

changing it to a different queue.changing it to a different queue.

Two Switching MethodsTwo Switching Methods Store-and-ForwardStore-and-Forward

The switch The switch receives the entire receives the entire frameframe, calculating the CRC at the , calculating the CRC at the end, before sending it to the end, before sending it to the destinationdestination

Cut-throughCut-through Fast forward switchingFast forward switching----only only

checks the destination MAC checks the destination MAC before immediately forwarding before immediately forwarding the framethe frame

Fragment FreeFragment Free--reads the first 64 --reads the first 64 bytes to reduce errors before bytes to reduce errors before forwarding the frameforwarding the frame

VLANs (IEEE 802.1q)VLANs (IEEE 802.1q) CharacteristicsCharacteristics

A A logical grouping of network deviceslogical grouping of network devices or users that or users that are not restricted to a physical switch segment. are not restricted to a physical switch segment.

The devices or users in a VLAN can be grouped by The devices or users in a VLAN can be grouped by function, department, application, and so on, function, department, application, and so on, regardless of their physical segment location. regardless of their physical segment location.

A VLAN creates a single broadcast domain that is not A VLAN creates a single broadcast domain that is not restricted to a physical segment and is treated like a restricted to a physical segment and is treated like a subnet. subnet.

VLAN setup is done in the switch by the network VLAN setup is done in the switch by the network administrator using the vendor’s software.administrator using the vendor’s software.

Institute of Technology,Sligo Dept of Computing

The Spanning-Tree ProtocolThe Spanning-Tree Protocol

Overview of STPOverview of STP Elements of the Spanning Tree ProtocolElements of the Spanning Tree Protocol

Main function of STP is to Main function of STP is to allow redundant paths in a allow redundant paths in a switched/bridged networkswitched/bridged network without incurring latency from without incurring latency from the effects of loops.the effects of loops.

STP prevents loops by calculating a stable spanning-STP prevents loops by calculating a stable spanning-tree network topology (similar to OSPF operation)tree network topology (similar to OSPF operation)

Spanning-tree frames (called bridge protocol data units--Spanning-tree frames (called bridge protocol data units--BPDUs) are sent and received by all switches in the BPDUs) are sent and received by all switches in the network and are used to determine the spanning-tree network and are used to determine the spanning-tree topologytopology

STP operation is covered in detail in Semester 7 of the STP operation is covered in detail in Semester 7 of the CCNP curriculum.CCNP curriculum.

Five STP StatesFive STP States States are established by configuring each port States are established by configuring each port

according to policyaccording to policy Then the STP modifies the states based on traffic Then the STP modifies the states based on traffic

patterns and potential loopspatterns and potential loops The The default order of STP states aredefault order of STP states are::

BlockingBlocking--no frames forwarded, BPDUs heard--no frames forwarded, BPDUs heard ListeningListening--no frames forwarded, listening for data frames--no frames forwarded, listening for data frames LearningLearning--no frames forwarded, learning addresses--no frames forwarded, learning addresses ForwardingForwarding--frames forwarded, learning addresses--frames forwarded, learning addresses DisabledDisabled--no frames forwarded, no BPDUs heard--no frames forwarded, no BPDUs heard