LAN Switching

1
LAN Switching

• Roy H. John
• Cisco Networking Academy
• Youngstown State University

2
LAN Switching

• Today, network designers are moving away from
  using bridges and hubs and are primarily using
  switches and routers to build networks.
• This chapter discusses problems in a local-area
  network (LAN) and possible solutions that can
  improve LAN performance.
• Finally, you will learn about Ethernet, Fast
  Ethernet, and VLANs, and the benefits of these
  technologies.

3
Three Main Factors that contribute to Network
Congestion

• Multitasking
• The multitasking environment present in current
  desktop operating systems allows for simultaneous
  network transactions.
• Faster operating systems
• Faster operating systems, being able to
  multitask, users are able to initiate
  simultaneous network transactions
• More Web-based applications.

4
LAN Communication problems

• The essential elements of Ethernet LANs can each
  contribute to network performance degradation.
• The broadcast nature of Ethernet, the use of
  CSMA/CD which only allows one station at a time
  to transmit.
• The use of multimedia applications.
• The normal latency of Ethernet media and related
  Layer 1, 2, and 3 devices.
• Finally the use of Layer 1 repeaters and hubs
  all of these are part of a normal Ethernet LAN,
  yet they can, in certain circumstances, become a
  problem.

5
Ethernet Half Duplex

• Ethernet is a half-duplex technology.
• Each Ethernet host checks the network to see
  whether data is being transmitted before it
  transmits additional data.
• If the network is already in use, the
  transmission is delayed.
• Despite transmission deferral, two or more
  Ethernet hosts can transmit at the same time,
  which results in a collision.
• When a collision occurs, the host that first
  detects the collision will send out a jam signal. 

6
Half- Duplex cont.

• Upon hearing the jam signal, each host will wait
  a random period of time before attempting to
  transmit.
• This random period of time is known as a backoff
  algorithm.
• As more hosts are added to the network and begin
  transmitting, collisions are more likely to
  occur.

7
Network Congestion

• The combination of more powerful computers and
  network-intensive applications has created a need
  for network capacity, or bandwidth, that is much
  greater than the 10 Mbps that is available on
  shared Ethernet/802.3 LANs.
• Today's networks are experiencing an increase in
  the transmission of large graphics files, images,
  full-motion video, and multimedia applications,
  as well as an increase in the number of users on
  a network.
• All these factors place an even greater strain on
  Ethernet's 10-Mbps bandwidth capacity.

8
Network Latency

• If we just consider the two hosts shown, there
  are at least 3 sources of latency.
• 1st, there is the time it takes the NIC to place
  voltage pulses on the wire and the time it takes
  the receiving NIC to interpret these pulses.
• 2nd, there is the time spent while CSMA/CD makes
  the stations take turns transmitting by requiring
  random backoff times.
• 3rd, there is the actual propagation delay as the
  signal takes time - albeit a very short time - to
  actually travel down the cable.

9
Ethernet 10base-T transmission time

• These transmission times are for 10 Mbps
  Ethernet different Ethernet speeds (100 Mbps,
  1000 Mbps) have different timing charts.
• Each 10 Mbps Ethernet bit has a 100 ns window for
  transmission. A byte is equal to 8 bits.
  Therefore, 1 byte takes a minimum of 800 ns to
  transmit.

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The Benefits of Using Repeaters

• Benefits of repeaters include extension of the
  length of the network
• An increased number of stations that can be
  connected.
• Also, the repeater concept can be expanded to the
  multiport repeater, or hub, which provides the
  benefits of repeaters plus connectivity between
  multiple devices.
• What are some Disadvantages????

11
Full Duplex Ethernet

• Full-duplex Ethernet allows the transmission of a
  packet and the reception of a different packet at
  the same time.
• This simultaneous transmission and reception
  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.
• Full-duplex Ethernet can use an existing shared
  medium as long as the medium meets minimum
  Ethernet standards. 

12
Full Duplex - continued

• To transmit and receive simultaneously, a
  dedicated port is required for each node.
• Full-duplex connections can use 10BASE-T,
  100BASE-TX, or 100BASE-FX media to create
  point-to-point connections. The network interface
  cards (NICs) on both ends need to have
  full-duplex capabilities.
• The full-duplex Ethernet switch takes advantage
  of the two pairs of wires in the cable.  This is
  done by creating a direct connection between the
  transmit (TX) at one end of the circuit and the
  receive (RX) at the other end. 
• With these two stations connected this way, a
  collision-free domain is created because the
  transmission and receipt of data occurs on
  separate non-competitive circuits.

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Full Duplex - continued

• Ethernet usually can only use 50-60 of the
  10-Mbps available bandwidth because of collisions
  and latency.
• Full-duplex Ethernet offers 100 of the bandwidth
  in both directions.
• This produces a potential 20-Mbps throughput-
  10-Mbps TX and 10-Mbps RX.

14
Why Segment LANs?
15
Why Segment LANs?
16
Segmenting with Bridges

• Ethernet LANs that use a bridge to segment the
  LAN provide more bandwidth per user because there
  are fewer users on each segment.
• Bridges "learn" a network's segmentation by
  building address tables that contain the address
  of each network device and which segment to use
  to reach that device. Bridges are Layer 2 devices
  that forward data frames according to the frames'
  Media Access Control (MAC) addresses.
• Bridges increase the latency in a network by
  10-30. This latency is due to the decision
  making required of the bridge or bridges in
  transmitting data.
• A bridge is considered a store-and-forward device
  because it must examine the destination address
  field in the frame prior to determining the
  interface to which the frame is forwarded.

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Segmenting with routers

• Routers connect different networks hence when
  you insert them in a LAN you are obviously
  causing segmentation.
• While this is one benefit of using routers, their
  main purpose remains best path selection and
  switching
• Protocols that require an acknowledgement from
  the receiver to the sender for every packet as it
  is delivered have a 30-40 loss of throughput.
  Protocols that require minimal acknowledgements
  (sliding-window protocols) suffer a 20-30 loss
  of throughput.

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Segmenting with LAN switches

• A switch can segment a LAN into microsegments,
  which are single host segments. This creates
  collision-free domains from one larger collision
  domain.
• Although the LAN switch eliminates collision
  domains, all hosts connected to the switch are
  still in the same broadcast domain. Therefore,
  all nodes connected through the LAN switch can
  see a broadcast from just one node.
• Switched Ethernet is based on Ethernet. Each node
  is directly connected to one of its ports or a
  segment that is connected to one of the switch's
  ports. This creates a 10-Mbps bandwidth
  connection between each node and each segment on
  the switch. A computer connected directly to an
  Ethernet switch is its own collision domain and
  accesses the full 10 Mbps.

19
Segmenting with LAN switches

• A LAN that uses a Switched Ethernet topology
  creates a network that behaves as though it has
  only two nodes-the sending node and the receiving
  node. These two nodes share the 10-Mbps bandwidth
  between them, which means that nearly all the
  bandwidth is available for the transmission of
  data. In a Switched Ethernet implementation, the
  available bandwidth can reach close to 100.
• Ethernet switching increases the bandwidth
  available on a network by creating dedicated
  network segments and connecting those segments in
  a virtual network within the switch. This virtual
  network circuit exists only when two nodes need
  to communicate.

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The 2 basic operations of a switch

• The two basic operations of a switch are
• Building and maintaining a switching table
• Essentially classifying Layer 2 MAC addresses as
  local to an interface or non-local to an
  interface
• Actually switching layer 2 frames.
• Comparisons could be made to routers, which build
  and maintain routing tables (mapping Layer 3
  addresses to the interface out which they are
  reachable) and actually switching the Layer 3
  packets.

21
Ethernet Switch Latency

• Each switch used on a 10 Mbps Ethernet LAN adds
  latency to the network.
• However, the type of switching used can help
  overcome the built-in latency of some switches. A
  switch between a workstation and a server adds 21
  microseconds to the transmission process. A
  1000-byte packet has a transmission time of 800
  microseconds. A packet sent from a workstation to
  a server has a total transmission time of 821
  microseconds (800 21 821).
• Because of the switching employed, known as cut-
  through, the MAC address of the destination
  device is read and the switch begins transmitting
  the packet before the packet completely arrives
  in the switch. This more than makes up for the
  inherent latency in the switch.

22
Layer 2 and Layer 3 Switching

• There are two methods of switching data
  frames-Layer 2 and Layer 3 switching.
• Switching is the process of taking an incoming
  frame from one interface and delivering it out
  through another interface. The difference between
  Layer 2 and Layer 3 switching is the type of
  information inside the frame that is used to
  determine the correct output interface. Layer 2
  switching does not look inside a packet for
  network-layer information as does Layer 3
  switching.
• Layer 2 switching looks at a destination MAC
  address within a frame. Layer 2 switching builds
  and maintains a switching table that keeps track
  of the MAC addresses that belong to each port or
  interface.

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Continues

• If the Layer 2 switch does not know where to send
  the frame, it broadcasts the frame out all its
  ports to the network to learn the correct
  destination.
• When the frame's reply is returned, the switch
  learns the location of the new address and adds
  the information to the switching table. They are
  unique addresses that are derived in two parts-
  the manufacturing (MFG) code and the unique
  identifier. The vendor assigns a unique
  identifier.
• In addition, Layer 2 addresses assume a flat
  address space with universally unique addresses.
  Layer 3 switching operates at the network layer.
  It examines packet information and forwards
  packets based on their network-layer destination
  addresses. Layer 3 switching also supports router
  functionality.

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Continued

• For the most part, the network administrator
  determines the Layer 3 addresses. By creating
  Layer 3 addresses, a network administrator
  creates local areas that act as single addressing
  units and assigns a number to each local entity.
  If users move to another building, their end
  stations obtain new Layer 3 addresses, but their
  Layer 2 addresses remain the same.
• Therefore, a routed network can tie a logical
  addressing structure to a physical
  infrastructure, for example, through TCP/IP
  subnets or IPX networks for each segment. Traffic
  flow in a switched network is therefore
  inherently different from traffic flow in a
  routed network.
• Hierarchical networks offer more flexible traffic
  flow than flat networks because they can use the
  network hierarchy to determine optimal paths and
  contain broadcast domains.

25
Microsegmentation

• An important point here is that the dedicated
  paths between sending and receiving hosts within
  the switch are temporary.
• The switches power comes from the fact that it
  can rapidly make and break these 1 to 1
  connections through its various ports, depending
  upon the data in its switching table.

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The Benefits of LAN switching

• A LAN switch allows many users to communicate in
  parallel through the use of virtual circuits and
  dedicated network segments in a collision-free
  environment.
• This maximizes the bandwidth available on the
  shared medium.
• Also, moving to a switched LAN environment is
  very cost-effective because you can reuse
  existing hardware and cabling.
• Finally, the power of the switch combined with
  the software to configure LANs give network
  administrators great flexibility in managing the
  network.

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Symmetric Switching

• A symmetric switch provides switched connections
  between ports with the same bandwidth, such as
  all 10-Mbps ports or all 100-Mbps ports.

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Asymmetric switching

• Asymmetric switching makes the most of
  client/server network traffic flows where
  multiple clients are communicating with a server
  at the same time, requiring more bandwidth
  dedicated to the switch port that the server is
  connected to in order to prevent a bottleneck at
  that port.
• Memory buffering in an asymmetric switch is
  required to allow traffic from the 100-Mbps port
  to be sent to a 10-Mbps port without causing too
  much congestion at the 10-Mbps port.

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2 switching methods

• Store-and-forward
• The entire frame is received before any
  forwarding takes place.
• Latency occurs while the frame is being received
• The latency is greater with larger frames because
  the entire frame takes longer to read.
• Error detection is high because of the time
  available to the switch to check for errors while
  waiting for the entire frame to be received.

30
Continued

• Cut-through
• The switch reads the destination address before
  receiving the entire frame. The frame is then
  forwarded before the entire frame arrives.
• This mode decreases the latency of the
  transmission and has poor LAN Switching error
  detection.
• Fast-forward and fragment-free are two forms of
  cut-through switching

31
What is a VLAN?

• An Ethernet switch physically segments a LAN into
  individual collision domains. However, each
  segment is still part of one broadcast domain.
• The total number of segments on a switch equals
  one broadcast domain. This means that all nodes
  on all segments can see a broadcast from a node
  on one segment.
• A VLAN is 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 software.
• VLANs are not standardized and require the use of
  proprietary software from the switch vendor.

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Overview of the Spanning-Tree protocol

• The main function of the Spanning-Tree Protocol
  is to allow duplicate switched/bridged paths
  without incurring the latency effects of loops in
  the network.
• Bridges and switches make their forwarding
  decisions for unicast frames based on the
  destination MAC address in the frame.
• If the MAC address is unknown, the device floods
  the frame out all ports in an attempt to reach
  the desired destination.
• It also does this for all broadcast frames. 

33
Continued

• The Spanning-Tree Algorithm, implemented by the
  Spanning-Tree Protocol, prevents loops by
  calculating a stable spanning-tree network
  topology.
• When creating fault-tolerant networks, a
  loop-free path must exist between all Ethernet
  nodes in the network.
• The Spanning-Tree Algorithm is used to calculate
  a loop-free path. Spanning-tree frames, called
  bridge protocol data units (BPDUs), are sent and
  received by all switches in the network at
  regular intervals and are used to determine the
  spanning-tree topology.

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The five Spanning-Tree states

• Blocking-No frames forwarded, BPDUs heard
• Listening-No frames forwarded, listening for
  frames
• Learning-No frames forwarded, learning addresses
• Forwarding-Frames forwarded, learning addresses
• Disabled-No frames forwarded, no BPDUs heard
• You can determine the status, cost, and priority
  of ports and VLANs by using the show spantree
  command.

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TCS Overview

• LAN Switching
• Describe the advantages of LAN segmentation.
• Describe LAN segmentation using bridges.
• Describe LAN segmentation using routers.
• Describe LAN segmentation using switches.
• Describe the benefits of network segmentation
  with bridges
• Describe the benefits of network segmentation
  with routers.
• Describe the benefits of network segmentation
  with switches.
• Name and describe two switching methods.
• Distinguish between cut-through and
  store-and-forward switching.
• Define and describe the function of a MAC
  Address.
• Ethernet
• Describe network congestion problem in Ethernet
  networks.  
• Describe full- and half-duplex Ethernet
  operation.  
• Describe the features and benefits of Fast
  Ethernet.
• Describe the guidelines and distance limitations
  of Fast Ethernet.