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Module 8Ethernet Switching

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Ethernet Switching

• Ethernet is a shared media– One node can transmit data at a time

• More nodes increases the demands on the available bandwidth – The probability of collisions increases, resulting in

more retransmissions

• A solution to the problem is to segment.

• Segmenting creates more collision domains

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Shared Media Environment

• Shared media environment –

– multiple hosts have access to the same medium

• Extended shared media environment –

– Using networking devices extends the environment to accommodate multiple access or longer cable distances

• Point-to-point network environment –

– one device is connected to only one other device (ex. dialup network connections)

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Shared media environments

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Layer 1 Devices• Layer 1 devices

– repeaters and hubs

• Extend collision domains

• Primary function is extending cable segments

• Additional hosts increase the amount of traffic

• More traffic = greater chances of collisions – This results in diminished performance

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Collision Domains

• Collision Domains

– Connected physical network segments where collisions can occur

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Collision domains

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Collision Domains

• The types of devices that interconnect the media segments define collision domains

• Classified as OSI Layer 1, 2 or 3 devices

• Layer 1 devices do not break up collision domains

• Layer 2 and Layer 3 devices break up collision domains– Increasing the number of collision domains is

known as segmentation

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Segmentation

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Network segment

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Layer 2 Devices

• Layer 2 devices

– Bridges and Switches

– Segments collision domains

– Controls frame propagation using the MAC address

– Tracks the MAC addresses and segment they are on

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Layer 2 Bridging

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Bridges

• Has only two ports and divides a collision domain into two parts

• Entire network will share the same logical broadcast address space

• All decisions made are based on MAC or Layer 2 addressing

• No effect on the logical or Layer 3 addressing

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Layer 2 Switching

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Switches

• A switch is a fast, multi-port bridge

• Each port creates its own collision domain

• A switch dynamically builds and maintains a Content-Addressable Memory (CAM) table

• The CAM holds all of the necessary MAC information for each port

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Switch Operation

• Communication in both directions at once is known as full duplex

• Most switches are capable of supporting full duplex, as are most network interface cards (NICs)

• In full duplex mode, there is no contention for the media.– A collision domain no longer exists– Theoretically, the bandwidth is doubled when

using full duplex

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Switch modes

Store and Forward

Cut through

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Switch Modes

• Cut-through switching

– A switch transfers the frame as soon as the destination MAC address is received

– lowest latency

– no error checking

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Switch Modes

• Store-and-forward switching

– Higher latency

– The switch receives the entire frame before sending it out

– Verifies the Frame Check Sum (FCS)

– Invalid frames are discarded at the switch

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Switch Modes

• Synchronous switching– The source port and destination port must

be operating at the same bit rate

• Asynchronous switching – 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

– Store-and-forward must be used  

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Switch Modes

• Asymmetric switching

– Switched connections between ports of unlike bandwidths

– Asymmetric switching is optimized for client/server

– A server requires more bandwidth dedicated to the server port to prevent a bottleneck at that port

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Layer 2 Broadcasts

• Ethernet Broadcasts

– When a node needs to communicate with all hosts on the network

– A broadcast frame with a destination MAC address 0xFFFFFFFFFFFF is sent

– The network interface card (NIC) of every host must respond

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Layer 2 Broadcasts

• The three sources of broadcasts and multicasts:

– Workstations

– Routers

– Multicast Applications

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Layer 3 Devices

• Layer 3 devices

– Routers

– Do not forward collisions

– Breaks up collision domains

– Broadcast domains are controlled

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Broadcast domain

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Broadcast Domain

• Broadcast Domain – A grouping of collision domains

– Broadcasts have to be controlled at Layer 3 devices

– Layer 2 and Layer 1 devices do not control broadcasts

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Data Flow

• Layer 2 devices filter data frames based on the destination MAC address

– A Layer 2 device will forward the frame unless something prevents it from doing so

• Layer 3 devices filter data packets based on IP destination address

– A Layer 3 device will not forward the frame unless it has to

– Layer 3 device creates multiple collision and broadcast domains

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Dataflow

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Latency

The delay between the time a frame leaves the source device and the time the frame reaches its destination

• The following conditions can cause delays:

– Physical media – Circuit delays

• Electronics that process the signal along the path – Software delays

• Decisions that must be made to implement switching and protocols

– Delays caused by the content of the frame• Destination MAC address has to be read

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Latency