Ethernet Switching

CCNA 1 CCNA 1

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Need for Switching

• When the number of devices trying to When the number of devices trying to access the network is low, the number of access the network is low, the number of collisions stays well within acceptable collisions stays well within acceptable limits. limits.

• Bridging was developed to help ease Bridging was developed to help ease performance problems that arose from performance problems that arose from increased collisions. increased collisions.

• Switching evolved from bridging Switching evolved from bridging

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Layer 2 Bridging/Switching

• A Bridge keeps a table of A Bridge keeps a table of MAC addressesMAC addresses and the and the associated portsassociated ports. The bridge then . The bridge then forwards or discards frames based on the forwards or discards frames based on the table entries.table entries.

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Bridge Operation

• The bridge has just been started so the bridge table is empty. The bridge just waits for traffic on the segment.

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Bridge Operation

• Host A pings Host B. Since the data is transmitted on the entire collision domain segment, both the bridge and Host B process the packet.

• The bridge adds the source address of the frame to its bridge table.

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Bridge Operation

• The destination address of the frame is checked against the bridge table. The address is not in the table

• Therefore the frame is forwarded to the other segment.

• The address of Host B has not been recorded yet.

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Bridge Operation

• Host B processes the ping request and transmits a ping reply back to Host A. The data is transmitted over the whole collision domain (Host A and the bridge).

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Bridge Operation

• The bridge adds the frame’s source address to its table.

• Since it received the frame on port 1, the bridge associates this address with port 1.

• As the destination address is already in the table, and is associated with port 1, the frame is not forwarded out port 2.

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Bridge Operation

• Host A pings Host C. Host B receives but discards the frame since it was not the intended destination.

• The bridge renews the source address of the frame to its bridge table.

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Bridge Operation

• Since the address is not in the table, the frame is forwarded to the other segment.

• The address of Host C has not been recorded yet.

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Bridge Operation

• Host C processes the ping request and replies back to Host A.

• The data is transmitted over the whole collision domain. Host D discards the frame since it is not the intended destination.

• The bridge adds the source address of the frame to its bridge table.

• The destination address is associated with Port 1, and is forwarded.

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Bridge Operation

• Host D pings Host C.

• The bridge adds the source address to its bridge table.

• As the destination address is on the same segment, the frame is not forwarded out port 1.

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Bridge Operation• Bridges make decisions based on _____ addresses, or layer __ Bridges make decisions based on _____ addresses, or layer __

addresses.addresses.• Bridges create _________ domains, which reduce traffic, but do Bridges create _________ domains, which reduce traffic, but do

not affect ________ , or layer __ addresses. not affect ________ , or layer __ addresses. • The entire network will be part of the same __________ domain.The entire network will be part of the same __________ domain.

MAC 2

COLLISIONLOGICAL 3

BROADCAST

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• A switch is a multi-port bridge.A switch is a multi-port bridge.• Each port on a switch is in its own collision Each port on a switch is in its own collision

domain. A 20 port switch will have 20 collision domain. A 20 port switch will have 20 collision domains.domains.

• Increasing the number of collision domains is Increasing the number of collision domains is known as known as micro-segmentationmicro-segmentation..

• A switch builds and maintains a A switch builds and maintains a CContent-ontent-AAddressable ddressable MMemory (emory (CAMCAM) table, which holds ) table, which holds the MAC information for each port. the MAC information for each port.

Layer 2 Switching

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Layer 2 Switching

• A switch dynamically builds and maintains a A switch dynamically builds and maintains a Content-Addressable Memory (CAM)Content-Addressable Memory (CAM) table, table, holding all of the necessary MAC information for holding all of the necessary MAC information for each port, using ASICs.each port, using ASICs.

• AnAn Application-Specific Integrated Circuit (ASIC) Application-Specific Integrated Circuit (ASIC) is a device that can be programmed to perform is a device that can be programmed to perform functions at high speeds. functions at high speeds.

• Operations that were done in software can now be Operations that were done in software can now be done in hardware using an ASIC. done in hardware using an ASIC.

• The use of these technologies greatly reduced the The use of these technologies greatly reduced the delays caused by software processing.delays caused by software processing.

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Full Duplex• Most switches and most network interface cards (NICs) are capable of

supporting full duplex.• In full duplex mode, there is no contention for the media. • Thus, with switched full-duplex transmission there are no collision domains

. • Theoretically, the bandwidth is doubled when using full duplex.

A switch uses full-duplex mode to provide full bandwidth between two nodes on a network.

A Switch:• Doubles bandwidth between nodes• Collision-free transmission• Two 10 or 100 Mbps data paths

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Switch Benefits

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Latency

• Latency is the Latency is the delaydelay between the time a frame first between the time a frame first starts to leave the source device and the time the first starts to leave the source device and the time the first part of the frame reaches its destination.part of the frame reaches its destination.

• Caused by:Caused by:• Media - the time it takes for a signal to move Media - the time it takes for a signal to move

through the mediathrough the media• Electronic processing of signal (circuit delay)Electronic processing of signal (circuit delay)• Software switching decisionsSoftware switching decisions• Content of frame – eg reading destination addressContent of frame – eg reading destination address

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• Time is the issue. Users want their Time is the issue. Users want their computers to work as fast as possible.computers to work as fast as possible.

• How a frame is switched to the destination How a frame is switched to the destination port is a trade off between latency and port is a trade off between latency and reliability.reliability.

• A switch has three different methods it can A switch has three different methods it can used in processing frames.used in processing frames.

Switch Modes

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Switch Modes

• Cut-through switchingCut-through switching —— A switch can start to A switch can start to transfer the frame as soon as the destination MAC transfer the frame as soon as the destination MAC address is received. This has the lowest latency.address is received. This has the lowest latency.• There is NO error checking.There is NO error checking.

• Store-and-forward Store-and-forward switchingswitching —— The switch can The switch can receive the entire frame before sending it out the receive the entire frame before sending it out the destination port. This gives the switch software an destination port. This gives the switch software an opportunity to verify the frame check sum (FCS). opportunity to verify the frame check sum (FCS).

• Store-and-forward mode must be used for Store-and-forward mode must be used for

asynchronous switching.asynchronous switching.  

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Symmetric vs. Asymmetric switching • SymmetricSymmetric – the source and destination – the source and destination

ports must have the same bit rate to keep ports must have the same bit rate to keep the frame intact.the frame intact.

• AsymmetricAsymmetric – the bit rates are not the – the bit rates are not the same, so the frame must be stored at one bit same, so the frame must be stored at one bit rate, before it is sent out at the other bit rate. rate, before it is sent out at the other bit rate. • Used with ports of different bandwidths, Used with ports of different bandwidths,

such as with a server in a client/server such as with a server in a client/server network. (needed to prevent bottlenecks)network. (needed to prevent bottlenecks)

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Symmetric vs. Asymmetric switching

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Symmetric vs. Asymmetric switching

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• Fragment-free modeFragment-free mode – a compromise – a compromise between cut-through and store-and-forward between cut-through and store-and-forward packet switching packet switching

• The first 64 bytes are read before it starts The first 64 bytes are read before it starts to send the frame out the destination port.to send the frame out the destination port.

• The CRC and the frame length are The CRC and the frame length are verified before forwarding the frame.verified before forwarding the frame.

Switch Modes

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Switch Modes

Start of frame

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Collision Domains

Broadcast Domains

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Shared Media Environments

• It is important to be able to identify a shared media environment, It is important to be able to identify a shared media environment, because collisions only occur in a because collisions only occur in a shared environmentshared environment..

Multiple hosts have access to the same medium; eg copper, fiber, air

Networking devices extend the network to accommodate multiple access or longer cable distances.

One device is connected to only one other device; eg dialup connection

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Collision Domains

• Collision domains are the connected physical Collision domains are the connected physical network segments where collisions can occur. network segments where collisions can occur.

• Collisions cause network inefficiency. Collisions cause network inefficiency.

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• The 5-4-3-2-1 rule requires that the The 5-4-3-2-1 rule requires that the following guidelines should not be following guidelines should not be exceeded: exceeded: • Five segments of network media Five segments of network media • Four repeaters or hubs Four repeaters or hubs • Three host segments of the network Three host segments of the network • Two link sections with no hosts Two link sections with no hosts • One large collision domainOne large collision domain

The Four Repeater Rule

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Collisions and Collision Domains

• When two bits are propagated at the same time on the same network, a collision will occur.• They are not inherently bad.They are not inherently bad.• They are a normal function of Legacy Ethernet.They are a normal function of Legacy Ethernet.

• Data on the network during a collision is lost and Data on the network during a collision is lost and usually must be retransmitted.usually must be retransmitted.• Increased collisions indicate congestion.Increased collisions indicate congestion.

• Collisions affect ALL nodes in a collision domain.Collisions affect ALL nodes in a collision domain.• Networks with only Layer 1 components are a Networks with only Layer 1 components are a

single collision domain.single collision domain.

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Shared Media, Repeaters, Hubs, and Collision Domains

Extended by a huband repeater

Each is a single collision domain!

Shared access

Extended by a repeater

Created by a hub

Extended by a repeater

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How many collision domains?

ONE

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How do you solve the problem?

Add a Bridge

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Add a bridge = 2 collision domains

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Segmentation

• Layer 2 devices segment or divide collision Layer 2 devices segment or divide collision domains. domains.

• Segmentation makes networks more Segmentation makes networks more efficient and allow data to be transmitted on efficient and allow data to be transmitted on different segments of the LAN at the same different segments of the LAN at the same time without the frames colliding. time without the frames colliding.

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Segmenting with Bridges

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Segmenting with Switches

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Segmenting with Routers

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Layer 2 Broadcasts = FFF-FFF-FFF-FFF

• The three sources of broadcasts and multicasts in The three sources of broadcasts and multicasts in IP networks are workstations, routers, and IP networks are workstations, routers, and multicast applications.multicast applications.• ARP requestsARP requests

• Routing table updatesRouting table updates

• Multicasts to specified IP addressesMulticasts to specified IP addresses

• Layer 2 devices must flood all broadcast and Layer 2 devices must flood all broadcast and multicast traffic. multicast traffic.

• The accumulation of broadcast and multicast The accumulation of broadcast and multicast traffic from each device in the network is referred traffic from each device in the network is referred to as broadcast radiation.to as broadcast radiation.

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The only devices that can segment collision domains are bridges, switches (both Layer 2), and routers (Layer 3).

Broadcast Domains?

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2 collision domains1 broadcast domain

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2 collision domains1 broadcast domain

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Summary

• Bridges and Switches are layer two devices. Bridges and Switches are layer two devices. Forward frames after examining destination Forward frames after examining destination MAC addressMAC address

• Each port of the switch has a separate Each port of the switch has a separate collision domain (microsegmentation).collision domain (microsegmentation).

• Small collision domain => less collisions.Small collision domain => less collisions.

• STP is used to prevent loops created by STP is used to prevent loops created by redundant links.redundant links.

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Collision and Broadcast Domains

• All decisions made by a bridge are based on All decisions made by a bridge are based on the ____ or ______ addressing and do not the ____ or ______ addressing and do not affect the _______ or ___________.affect the _______ or ___________.

• A bridge will create A bridge will create more ____________more ____________ but but will not add _____________.will not add _____________.

MAC Layer 2

Layer 3 Logical address

Collision domains

Broadcast domains

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Switching/Bridging Table

E0

E0

E1

E1

0260.8c01.1111

0260.8c01.2222

0260.ec01.3333

0260.8c01.4444

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Fill in the blanks

Bridge Bridge SwitchSwitch

Number of portsNumber of ports

Number of Number of collision domainscollision domains

Number of Number of broadcast domainsbroadcast domains

Table nameTable name

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22

1 11

MultipleMultiple

MultipleMultiple

Bridging TableBridging Table Content-Addressable Content-Addressable memory (CAM)memory (CAM)

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How many collision/broadcast domains?

Collision =

Broadcast =

6

2

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• What impact does a bridge have on a What impact does a bridge have on a network? network? • Breaks up collision domainsBreaks up collision domains

• What happens with switched full-What happens with switched full-duplex transmissions? duplex transmissions? • No collisions domainsNo collisions domains

• What is STP and what 2 tasks does it What is STP and what 2 tasks does it perform? perform? • Spanning Tree ProtocolSpanning Tree Protocol• elect root bridge & decide designated elect root bridge & decide designated

portsports

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Broadcast Domains1.1. A broadcast domain is a grouping of collision A broadcast domain is a grouping of collision

domains that are connected by ________ devices. domains that are connected by ________ devices.

2.2. Smaller collision domains increase the opportunity Smaller collision domains increase the opportunity for each host in the network to for each host in the network to ____________________. ____________________.

3.3. _________ are forwarded by Layer 2 devices and if _________ are forwarded by Layer 2 devices and if excessive, can reduce the efficiency of the entire excessive, can reduce the efficiency of the entire LAN.LAN.

4.4. Broadcasts have to be controlled at Layer __.Broadcasts have to be controlled at Layer __.

5.5. _______ do not forward broadcasts. _______ do not forward broadcasts. 

6.6. Layer 3 forwarding is based on the destination Layer 3 forwarding is based on the destination __________ and not the MAC address. __________ and not the MAC address.

Layer 2

gain access to the media

Broadcasts

Routers

3

IP address

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How many collision domains are there?How many collision domains are there?How many broadcast domains are there?How many broadcast domains are there?

Segmentation166

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• Breaking up collision domains is calledBreaking up collision domains is called __________________________

• What would relieve the problems of excessive What would relieve the problems of excessive collisions? collisions? • Reduce collisions domains & replace hubs Reduce collisions domains & replace hubs

with switches or bridgeswith switches or bridges• What is the area that controls frame What is the area that controls frame

propagation within that area? propagation within that area? • Collision domainCollision domain

• What is the only device that breaks up What is the only device that breaks up broadcast domains? Describe how it works at broadcast domains? Describe how it works at each layer? each layer? • Routers—do not forward broadcastsRouters—do not forward broadcasts

• What does a router use to make forwarding What does a router use to make forwarding decisions? decisions? • Destination IP addressDestination IP address

segmentationsegmentation

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• Name the four devices above, the layer of the OSI Name the four devices above, the layer of the OSI model they operate at, and the method they base model they operate at, and the method they base their addressing on to forward packets on a network.their addressing on to forward packets on a network.

Bridge layer 2 FrameBridge layer 2 Frame

Repeater layer 1 BitsRepeater layer 1 Bits

Router layer 3 PacketRouter layer 3 Packet

Switch layer 2 FrameSwitch layer 2 Frame

THAT’S ALL FOLKS!