LAN Switching

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LAN Switching
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Table of Contents

• LAN Communication Problems

Go There!

• Full-Duplex, Fast Ethernet, and Segmentation

Go There!

• Switching and VLANs

Go There!

• The Spanning-Tree Protocol

Go There!
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LAN Communication Problems
Table of Contents
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Network Performance

• Network congestion has increased significantly
  since the mid-90s due to
• Multitasking Operating Systems
• multiple simultaneous network transactions (e.g.,
  ftp download browsing)
• Faster Processing Power
• 1980s 1 MIPS Today over 75 MIPS
• Network-intensive Applications
• accessing network servers to use applications,
  files, etc.

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Elements of Ethernet/802.3

• Characteristics
• Most common LAN architecture
• Used to transport data between devices connected
  to the same delivery medium
• Uses a data frame broadcast method
• Negative effects of a shared LAN
• broadcast delivery of all frames
• CSMA/CD collisions are inherent
• distance limitation requires using repeaters to
  extend

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Half-Duplex Ethernet

• Properties
• Only one host can transmit at a time because the
  NIC needs to listen for collisions
• The NIC provides several circuits. Most
  important are
• receive (RX), transmit (TX), and collision
  detection
• bandwidth usage 50 to 60

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CSMA/CD

• Operation
• Devices on shared media listen for a carrier
  before transmitting
• If no carrier is sensed for a specific period of
  time, a device can transmit
• If two devices transmit simultaneously, a
  collision occurs. The NIC senses this because it
  is transmitting and receiving at the same time
• The first device to detect the collision will
  generate a jam signal (colliding devices continue
  to transmit so that all devices will hear the
  collision)
• All devices calculate a backoff algorithm which
  will delay transmission for a random length of
  time.
• First device whos delay time expires can attempt
  to transmit data.

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Network Congestion

• Occurs as more people utilize a network to...
• Share large files (e.g. databases, applications,
  etc.)
• Access file servers
• connect to the Internet
• Relieving congestion requires
• Increasing the amount of bandwidth and/or
• Using available bandwidth more efficiently

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Network Latency

• Latency explained
• Represents the time it takes a frame to travel
  from is source device to its final destination on
  the network (also know as propagation delay)
• Latency can also be described as the delay
  between the time a device requests access to a
  network and the time it is granted permission to
  transmit
• For switches and routers, latency is the amount
  of delay between the time when the device
  receives the frame on one interface and forwards
  that frame out another interface
• Routers have more inherent latency than a switch.
  Why?

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Ethernet Transmission Time

• Defined
• Transmission time is the time necessary to move a
  packet from the data link layer to the physical
  layer
• 10BaseT Transmission Time
• Each bit has a 100ns window for transmission
• ns-nanosecond (1 billionth of a second)
• So each byte has what size window?
• A 64 byte frame (the smallest allowed frame)
  requires 51,200 ns or 51.2 microseconds
• Just to frame a 1000 byte packet requires 800
  microseconds
• Additional latency will be added propagating the
  frame down the wire and by any additional devices
  the frame has to go through before reaching the
  destination

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Using Repeaters

• What is attenuation?
• Loss of signal strength as it travels through the
  network caused by resistance inherent in the
  medium
• Benefits of Using a Repeater
• a layer 1 device that cleans up and boosts the
  signal
• extends the coverage area of a LAN segment
• Negative Effects of Using a Repeater
• increases the collision domain size
• increases the broadcast domain size
• cant filter traffic based on Layer 2 or 3
  addressing

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Full-Duplex, Fast Ethernet,and Segmentation
Table of Contents
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Full-Duplex Ethernet

• Simultaneous TX and RX
• 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 in the
  cable and a switched connection between each
  node.
• this connection is considered point-to-point and
  is collision free.
• because both nodes can transmit and receive at
  the same time, there are no negotiations for
  bandwidth.
• 100 of bandwidth is available 10 Mbps increases
  to 20 Mbps of potential throughput (10 Mbps TX
  10 Mbps RX)

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LAN Segmentation

• Benefits of Segmenting the Network

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

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Segmenting with Bridges

• Bridge Operation
• Bridges learn a networks segmentation by
  building address tables that contain
• Bridge interface that will reach that device
• Each devices MAC address

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Segmenting with Bridges

• Generic Frame Format
• Frame can be any length depending on technology
• Ethernet frame can be up to 1522 bytes long
• Address section is 12 bytes (6 bytes for each
  MAC)
• FCS contain the CRC to check frame for errors

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Segmenting with Bridges

• Bridge Performance
• adds 10 to 30 latency due to decision-making
  process
• considered a store-and-forward device because it
  must calculate the CRC at the end of the frame to
  check it for errors before forwarding
• if the bridge does not have an entry for the
  destination MAC, it...
• adds the source MAC to its bridging table
• forwards the frame out all interfaces except the
  one it was received on
• when a reply returns, it adds the destination MAC
  to the table

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Segmenting with Routers

• Router Operation
• Routers...
• use layer 3 addressing (IP, IPX) and routing
  protocols (RIP, IGRP) to determine the path and
• switch the packet out the correct interface to
  the destination
• because a router must open the packet to read
  Layer 3 addressing, it adds latency
• In addition, protocols like TCP which require
  acknowledgments of every packet can increase
  latency, reducing throughput from 20 to 40

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Segmenting with Routers

• Router Benefits
• Like switches, routers segment collision domains.
• However, since a router will not forward
  broadcasts, it also segments broadcast domains.
• Each router interface represents its own
  broadcast domain.

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Segmenting with Switches

• Switching Benefits
• a switch is simply a multi-port bridge, making
  forwarding decisions based on MAC addresses
• so, like a bridge, segmenting a LAN with a switch
  creates more collision domains
• replacing hubs with switches therefore decreases
  congestion and increases available bandwidth.
• a switch can microsegment a LAN creating
  collision-free domains but still be in the same
  broadcast domain.
• switch creates a virtual circuits, allowing many
  users to communicate in parallel.

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Switching and VLANs
Table of Contents
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Switch Operation

• Switches perform two basic functions
• Building and maintaining switching tables
  (similar to a bridge table) based on MAC
  addresses
• Switching frames out the interface to the
  destination
• Differences between switches bridges
• Switches operate at higher speeds
• Switches are capable of creating virtual LANs
  (VLANs) through microsegmentation
• Bridges switch using software switches typically
  switch using hardware (called the switch fabric)

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Switch Latency

• A switch adds 21 microseconds of latency.
• This can be reduced by using a different
  switching method
• As opposed to store-and-forward, the switch can
  use cut-through switching which switches the
  packet as soon as the destination MAC is read.

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How a LAN Switch Learns Addresses

• MAC addresses are learned dynamically and are
  stored in CAM (content-addressable memory)
• Each time a switch stores an address entry in the
  table, it is time-stamped.
• The time-stamp is updated each time a frame is
  received
• Addresses whose time-stamp expires are deleted
  from the table
• This keeps switching tables small

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Benefits of LAN Switching

• Cost-effective switches only cost 3 to 5 times
  that of a hub
• Allows the creation of virtual circuits
• More flexibility in managing the network
• Reduces number of collisions
• Works with existing 802.3 cabling

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Symmetric Switching

• symmetric switching provides switched connections
  between ports with the same bandwidth (10/10 Mbps
  or 100/100 Mbps)
• can cause bottlenecks as users try to access
  servers on other segments.

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Asymmetric Switching

• asymmetric switching reduces the likelihood of a
  potential bottleneck at the server by attaching
  the segment with the server to a higher bandwidth
  port (100 Mbps)
• asymmetric switching requires memory buffering in
  the switch

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Memory Buffering

• Defined
• Area of memory in a switch where destination and
  transmission data are stored until it can be
  switched out the correct port.
• Two types
• Port-based memory buffering
• packets are stored in a queue on each port
• possible for one packet to delay transmission of
  other packets because of a busy destination port
• Shared memory buffering
• common memory buffering shared by all ports
• allows packets to be RX on one port and TX out
  another port without changing it to a different
  queue.

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Two Switching Methods

• Store-and-Forward
• The switch receives the entire frame, calculating
  the CRC at the end, before sending it to the
  destination
• Cut-through
• Fast forward switching--only checks the
  destination MAC before immediately forwarding
  the frame
• Fragment Free--reads the first 64 bytes to reduce
  errors before forwarding the frame

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VLANs (IEEE 802.1q)

• Characteristics
• A logical grouping of network devices or users
  that are not restricted to a physical switch
  segment.
• The devices or users in a VLAN can be grouped by
  function, department, application, and so on,
  regardless of their physical segment location.
• A VLAN creates a single broadcast domain that is
  not restricted to a physical segment and is
  treated like a subnet.
• VLAN setup is done in the switch by the network
  administrator using the vendors software.

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The Spanning-Tree Protocol
Table of Contents
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Overview of STP

• Elements of the Spanning Tree Protocol
• Main function of STP is to allow redundant paths
  in a switched/bridged network without incurring
  latency from the effects of loops.
• STP prevents loops by calculating a stable
  spanning-tree network topology (similar to OSPF
  operation)
• Spanning-tree frames (called bridge protocol data
  units--BPDUs) are sent and received by all
  switches in the network and are used to determine
  the spanning-tree topology
• STP operation is covered in detail in Semester 7
  of the CCNP curriculum.

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Five STP States

• States are established by configuring each port
  according to policy
• Then the STP modifies the states based on traffic
  patterns and potential loops
• The default order of STP states are
• Blocking--no frames forwarded, BPDUs heard
• Listening--no frames forwarded, listening for
  data frames
• Learning--no frames forwarded, learning addresses
• Forwarding--frames forwarded, learning addresses
• Disabled--no frames forwarded, no BPDUs heard

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Required Labs for this Chapter

• Spend your lab time completing all four labs in
  this Chapter
• Lab 2.3.7--Switching Characteristics
• Lab 2.3.10.1--Switch Management Console
• Lab 2.3.10.2--Switch Port Options
• Lab 2.4.2--Switch browser configuration
• Recommendation
• DO NOT TAKE THE TEST UNTIL YOUVE COMPLETED THE
  LABS!!

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Table of Contents
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