Ethernet Switches

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Ethernet Switches
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Switch Operation
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Repeater In Star Topology
Repeater
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Switch In Star Topology
Switch
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Switches

• Switch can create multiple, simultaneous virtual
  connections between nodes
• Like a telephone switch multiple calls can occur
  between different people at the same time
• This allows each node to have a dedicated
  connection to the network instead of a shared
  connection

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Switches
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Switch Components

• Ports
• Ports on a single switch can have different
  speeds or use different media
• Frame buffers
• Frames may be stored temporarily for various
  reasons in memory called frame buffers
• Queues
• Frames are queued while waiting to be processed
  or transmitted
• Backplane bus
• The circuitry that connects the ports

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Switch Architecture
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Switch Operation
Backplane
Ports
Two frames arrive simultaneously on different
ports
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Switch Operation

• Learning
• Switches examine the source MAC address of each
  frame and build a source address table (SAT) so
  they can forward frames correctly
• Forwarding
• When a frame arrives, the switch looks up its
  destination MAC address in the SAT to determine
  what to do with it

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Switch Learning Addresses
Source Address Table (SAT) Address
Port
1 2 3 4
Ports
AB 1
AF 4
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Forwarding Decision

• Broadcast
• Forwarded out every port except the one it
  arrived on
• Acts like a layer 1 repeater
• Unicast
• If the destination address is in the SAT then
• If the address is on a different port than the
  one the frame arrived on, the frame is forwarded
  out the correct port
• If the address is on the same port than the one
  the frame arrived on, the frame is dropped or
  filtered
• If the destination address is not in the SAT then
  the frame is treated like a broadcast

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Switch Forwarding Frames
Source Address Table (SAT) Address
Port
1 2 3 4
Ports
AB 1
AF 4
AC 2
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Forwarding Mechanisms

• Store and Forward
• The switch loads the frame into memory and checks
  the frame for errors
• Cut through
• The switch immediately forwards the frame without
  checking for errors
• Modified cut through
• The switch examines the first 64 bytes of the
  frame for errors

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Collision Domains
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Ethernet Collision Domains
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Fast Ethernet Diameters
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Switches and Collisions

• Collisions cannot occur in the switch
• Calculation of collision domain diameter begins
  and ends at switches

200 meters
Switch
100 meters
Repeater
Collision Domain A
Collision Domain B
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Full-Duplex

• Switches allow connections to be full-duplex
• Each individual port can be configured
  independently
• Full-duplex operation allows communication in
  both directions simultaneously
• CSMA/CD is no longer relevant in full-duplex
  operation
• A full-duplex Fast Ethernet connection has a
  total potential throughput of 200 Mbps

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Full-Duplex

• Benefit of full-duplex depends in part on the
  extent that communication is equal in both
  directions
• Clients will usually only see a 10 improvement
• Switch to switch links can see Up to 100
  improvement
• High potential benefits for real time
  applications
• Since collisions are not possible, variable
  latency is reduced

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Scalability
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Switch Uplinks

• Switches permit different speeds on different
  ports
• High speed ports are called uplink ports
• Frames coming from faster ports are stored in
  frame buffers temporarily until slower ports can
  catch up
• This allows connections with high traffic demands
  to have a higher data rates
• For example, connections to servers

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Server in Shared LAN
Most of the traffic in client/server networks is
directed at servers or to Internet connections.
Server
Repeater
In a shared media LAN only one client can
access the server at a time.
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Server in Switched LAN
Replacing the repeater with a switch doesnt
necessarily help by itself since most of the
traffic is still going over the link to the server
Server
Switch
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Shared vs. Switched LAN
Switched 10BASE-T throughput increases until the
100 Mbps connection to the server is saturated.
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Shared vs. Switched LAN
Switched 10BASE-T throughput increases until both
100 Mbps connections are saturated.
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Switch Throughput

• The throughput capacity of the switch depends on
  the capacity of the backplane
• Consider a 20 port Fast Ethernet ports
• Maximum throughput occurs of 10 ports are
  transmitting to the other 10 ports
• The backplane must be able to transfer 10 100
  Mbps or 1 Gbps
• If it cant, the switch is a blocking switch
  meaning that ports will be temporarily blocked
  because the switch cannot transmit data across
  its backplane fast enough to keep up

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Link Aggregation

• Some switches allow multiple ports to be combined
  to form a higher speed channel or trunk
• Two Fast Ethernet ports could be combined to form
  a 200 Mbps channel

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Flow Control

• Flow control is required when a node connected to
  a high speed port is transmitting to a node with
  a low speed port
• Flow control allows the receiving node or switch
  to tell the sender to slow down or stop
  temporarily
• In half-duplex mode, the receiver can cause a
  collision
• In full-duplex mode, the receiver can send a
  PAUSE frame (IEEE 802.3x)

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Spanning Tree Protocol
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Spanning Tree Protocol
Switch A
Loops can cause problems for Layer 2 devices.
Assume the workstation in the figure issues a
broadcast frame. Both Switch A and B will
forward the broadcast frame to LAN 1. When the
broadcast frame from Switch B arrives at Switch A
on LAN 1, Switch A will forward it back to LAN 2.
The same thing will happen at Switch B. The
frames will continue to circulate and broadcasts
will accumulate until a broadcast storm will
occur stopping the network.
LAN 1
LAN 2
Switch B
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Spanning Tree Protocol

• Using the STP, switches will communicate with one
  another using Bridge Protocol Data Units (BPDU)
• The switches will disable ports on selected
  switches until the network is organized in a
  logical tree topology
• Tree topologies dont have loops
• If conditions change the switches can recalculate
  the topology

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Spanning Tree
Usually, the root bridge should be the one which
has the highest traffic levels.
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Using Switches to Create VLANs

• Virtual local area networks (VLANs)
• Network within a network that is logically
  defined by grouping its devices switch ports in
  the same broadcast domain
• VLANs act as if they are separate physical
  networks
• Broadcast domain
• Combination of ports that make up a Layer 2
  segment and must be connected by a Layer 3 device

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Network Hardware and the OSI Model