CCNA 1 Module 8

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CCNA 1 Module 8

• Ethernet Switching

"Success means having the courage, the
determination, and the will to become the person
you believe you were meant to be
- Dr. George Sheehan
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Module 8 Objectives

• After completing this module you should be able
  to
• Define bridging and switching
• Define and describe the content-addressable
  memory (CAM) table
• Define latency
• Describe store-and forward and cut-through
  switching modes
• Explain Spanning-Tree Protocol (STP)
• Define collisions, broadcasts, collision domains,
  and broadcast domains
• Identify Layer 1, 2, and 3 devices used to create
  collision and broadcast domains
• Discuss data flow and problems with broadcasts
• Explain network segmentation and list devices
  used to create segments

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Layer 2 Bridging

• Ethernet shares media, only one node transmits at
  a time
• Add nodes increases demands (load) on available
  bandwidth media
• Break large segments into smaller parts and
  separate parts into isolated collision domains
• Bridges keep tables of MAC addresses and
  associates ports with source MACs
• Bridges forward or discard frames based on
  bridging table entries

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Layer 2 Bridging

• When the bridge starts, its table is empty.
• As traffic crosses the segment, it is processed
  by the bridge.
• If Host A pings Host B, data is transmitted on
  the entire collision domain segment, so both the
  bridge and Host B process the packet.

The bridge adds the source address of the frame
to its bridge table. Since the address was in
the source address field and the frame was
received on port 1, the frame must be associated
with port 1 in the table.
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Layer 2 Bridging

• Both Host A and the bridge receive the frame and
  process it

The bridge adds the source address of the frame
to its bridge table. Since the source address
was not in the bridge table and was received on
port 1, the source address of the frame must be
associated with port 1 in the table.
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Layer 2 Bridging

• Host A pings Host C
• Since the data is transmitted on the entire
  collision domain segment, both the bridge and
  Host B process the frame
• Host B discards the frame as it was not the
  intended destination

The bridge adds the source address of the frame
to its bridge table. Since the address is
already entered into the bridge table the entry
is just renewed.
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Layer 2 Bridging

• Both Host D and the bridge receive the frame and
  process it
• Host D discards the frame, as it was not the
  intended destination

Bridge adds source address of the frame to its
bridge table Since the address was in the
source address field and the frame was received
on port 2, the frame must be associated with port
2 in the table.
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Layer 2 Bridging - Switching

• Bridges commonly have two ports, dividing a
  collision domain into two parts
• Bridge decisions use Layer 2 addressing only
• Bridges divide collision domains but DO NOT
  effect broadcast domains
• Only layer 3 devices (router) can break up a
  broadcast domain
• Bridges create more collision domains but not
  more broadcast domains
• Switches are fast, multi-port bridges
• Each port creates its own collision domain
• Switches dynamically build maintain a
  Content-Addressable Memory (CAM) table

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Switch operation

• A switch is simply a bridge with many ports
• When only one node is connected to a switch port,
  the collision domain on the shared media contains
  only two nodes.
• The two nodes in this small segment, or collision
  domain, consist of the switch port and the host
  connected to it.
• These small physical segments are called
  microsegments

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Switch operation

• In a network that uses twisted-pair cabling, one
  pair is used to carry the transmitted signal from
  one node to the other node
• A separate pair is used for the return or
  received signal.
• It is possible for signals to pass through both
  pairs simultaneously
• The capability of communication in both
  directions at once is known as full duplex

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Switch operation

• 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
• Collision domains no longer exists
• The bandwidth is doubled when using full duplex

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Switch Operation

• CAM allows switch to directly find port
  associated with a MAC address without using
  search algorithms
• Application-specific integrated circuit (ASIC)
  consists of undedicated logic gates programmable
  to perform functions at logic speeds
• These technologies greatly reduce software delays
  and enabled a switch to keep pace with demands of
  many microsegments and high bit rates

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Latency

• Latency delay between time a frame first starts
  to leave source device and time first part of the
  frame reaches its destination
• Multiple conditions cause delays
• Media delays caused by finite speed signals can
  travel over physical media
• Circuit delays caused by electronics processing
  signal
• Software delays caused by software
  decision-making to implement switching/protocols.
• Delays caused by frame payload, where in the
  frame switching decisions are made

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Switch modes

• How a frame is switched to the destination port
  is a trade off between latency reliability
• Three Types
• Cut-through
• Fragment-free
• Store-and-forward
• Error Sensing (non CCNA)

Both cut-through and fragment-free have a fixed
latency. Store-and-forward has the highest
latency. Cut-through has the lowest latency.
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Cut-through Switching

• Start to transfer the frame as soon as the
  destination MAC address is received
• Results in the lowest latency
• But no error checking is available

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Store-and-forward Switching

• Receive the entire frame before sending it out
  the destination port
• Verifies the Frame Check Sum (FCS) to ensure that
  the frame was reliably received before sending it
  to the destination
• If the frame is found to be invalid, it is
  discarded at this switch rather than at the
  ultimate destination

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Fragment-free Switching

• A compromise between the cut-through and
  store-and-forward modes
• Fragment-free reads the first 64 bytes, which
  includes the frame header, and switching begins
  before the entire data field and checksum are
  read
• This mode verifies the reliability of the
  addressing and Logical Link Control (LLC)
  protocol information to ensure the destination
  and handling of the data will be correct

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Synchronous Switching

• All ports operate at the same bit rate
• When using cut-through methods of switching, both
  the source port and destination port must operate
  at the same bit rate in order to keep the frame
  intact
• Symmetric switching provides switched connections
  between ports of like bandwidth, for example all
  100Mbps

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Asynchronous Switching

• The ports operate at different bit rates
• If 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 mode must be used for
  asynchronous switching 

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Asymmetric Switching

• Asymmetric switching provides switched
  connections between ports of unlike bandwidths,
  such as a combination of 100 Mbps and 1000 Mbps
• Asymmetric switching is optimized for
  client/server traffic flows in which multiple
  clients simultaneously communicate with a server,
  requiring more bandwidth dedicated to the server
  port to prevent a bottleneck at that port

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Crossbar Switch Architecture

• Crossbar switch architecture cross connects each
  port to all the other ports via the backplane.
  This means that the switch requires N x N
  connections, adding to the complexity of the
  switch. Because there is a direct connection
  between all ports, traffic can be forwarded
  directly to multiple ports simultaneously. All
  ports receive a copy of the inbound frame, but
  not all ports are permitted to transmit the frame
  to the wire. A complex bus arbitration algorithm
  is needed to make this architecture work
• Crossbar architecture has a small problem. When a
  crossbar switch serves multiple networks, and two
  frames enter the switch at the same time destined
  for different ports, one of the frames is blocked
  while the first frame is forwarded. This results
  in all frames being queued as they flow through
  the switch. If there is sufficient traffic and
  insufficient buffer space on the switch, packets
  are dropped
• This problem is called Head of Line Blocking, and
  is a common problem with crossbar switches

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Hierarchical Switch Architecture

• Hierarchical switches differ from crossbar
  switches in that they do not use a mesh or
  crossbar. Instead, the switching is performed in
  a series of hierarchical connections allowing
  multiple connections between ports to be made
  simultaneously

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Spanning-Tree Protocol

• Networks are often designed with redundant paths
  for reliability fault tolerance
• While desirable, they can have undesirable
  effects, switching loops are one such side effect
  
• Switching loops can occur by design or by
  accident, and they can lead to broadcast storms
  that will rapidly overwhelm a network.
• Switches are provided with a standards-based
  protocol called the Spanning-Tree Protocol (STP)
• Each switch in a LAN using STP sends special
  messages called Bridge Protocol Data Units
  (BPDUs) out all its ports to let other switches
  know of its existence and to elect a root bridge
• Switches use the Spanning-Tree Algorithm (STA) to
  resolve and shut down the redundant paths

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Spanning-Tree Protocol States

• Each port on a switch using Spanning-Tree
  Protocol exists in one of the following five
  states
• Blocking
• Listening
• Learning
• Forwarding
• Disabled
• Port moves through five states as follows
• From initialization to blocking
• From blocking to listening or to disabled
• From listening to learning or to disabled
• From learning to forwarding or to disabled
• From forwarding to disabled

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Shared Environments

• All hosts directly connected at Layer 1
• Shared media environment multiple hosts with
  access to same medium
• Extended shared media environment special
  environment using networking devices to extend
  environment to accommodate multiple access or
  longer cable runs
• Point-to-point network environment used in
  dialup network connections, (home-use) shares
  networking environment. Host is connected to only
  one other device (modem and a phone line)

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Collision Domains

• Collision domains are connected physical network
  segments where collisions can occur
• When a collision happens on a network, all
  transmission stops
• This time-out period of time without
  transmissions varies determined by back-off
  algorithm for each network device 
• Layer 1 devices do NOT break up collision
  domains, Layer 2 and 3 devices do break up
  collision domains
• Layer 1 devices, (repeaters hubs) extend the
  Ethernet cable segments and collision domains
• Each added host increases the amount of potential
  traffic. Since Layer 1 devices pass on
  everything, the more traffic transmitted within a
  collision domain, the greater the chances of
  collisions
• Results Diminished network performance,
  especially if computers on the network are
  demanding large amounts of bandwidth

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Four-Repeater Rule

• Four repeater rule in Ethernet states no more
  than four repeaters (repeating hubs) can be
  between any two computers on the network
• Repeater latency, propagation delay, and NIC
  latency contribute to the four repeater rule
• 5-4-3-2-1 rule requires the following guidelines
  NOT be exceeded
• Five segments of network media
• Four repeaters or hubs
• Three host segments of the network
• Two link sections (no hosts)
• One large collision domain

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Segmentation

• Networking professional must have the ability to
  recognize collision domains
• Layer 2 devices segment or divide collision
  domains
• Layer 2 devices, map the MAC and which segment
  they are on
• Therefore these devices can control the flow of
  traffic at Layer 2, making networks more
  efficient
• Bridges and switches, effectively break up the
  collision domain into smaller parts (separate
  collision domains)
• Layer 3 devices, do not forward collisions,
  therefore use Layer 3 devices in a network to
  break up collision domains into smaller domains

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Layer 2 Broadcasts

• To communicate across all collision domains,
  protocols use broadcast and multicast frames at
  Layer 2
• To communicate with all hosts on a network, host
  sends a broadcast frame with a destination MAC
  address 0xFFFFFFFFFFFF, (requires all NICs to
  respond
• Layer 2 devices must flood broadcast and
  multicast traffic, this accumulation of broadcast
  and multicast traffic from each device in the
  network is referred to as broadcast radiation
• Circulation of broadcast radiation can saturate
  the network so there is no bandwidth left for
  applications
• In this case, new network connections cannot be
  established, and existing connections may be
  dropped, resulting a broadcast storm

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Broadcast Domains

• A broadcast domain is a grouping of collision
  domains connected by Layer 2 devices
• Broadcasts are controlled at Layer 3, since
  routers do not forward broadcasts. 
• For packets to be forwarded through a router it
  has already been processed by a Layer 2 device
  and frame data is stripped off
• Layer 3 forwarding is based on destination IP,
  NOT the MAC address
• For a packet to be forwarded it must contain an
  IP address outside the range of addresses
  assigned to the LAN (particular router interface)
  and router must have a destination to send the
  specific packet to in its routing table

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Introduction to Data Flow

• Data flow (collision and broadcast domains
  context) refers to how data frames propagate
  through a network.
• Focuses on the movement and encapsulation of data
  through Layer 1, 2 and 3 devices
• Data is encapsulated at the network layer with an
  IP source and destination address, and at the
  data-link layer with a MAC source and destination
  address
• Layer 1 devices do not filter, only repeat
  everything
• Layer 2 devices filter data frames based on the
  destination MAC address
• Layer 3 devices filter data packets based on IP
  destination address
• Layer 1 is used for media transmission , Layer 2
  for collision domain management, and Layer 3 for
  broadcast domain management

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Data Flow Through a Network
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Module 8 Summary

• By now you should be able to
• Define bridging and switching
• Define and describe the content-addressable
  memory (CAM) table
• Define latency
• Describe store-and forward and cut-through
  switching modes
• Explain Spanning-Tree Protocol (STP)
• Define collisions, broadcasts, collision domains,
  and broadcast domains
• Identify Layer 1, 2, and 3 devices used to create
  collision and broadcast domains
• Discuss data flow and problems with broadcasts
• Explain network segmentation and list devices
  used to create segments

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Questions???
Now identify the separate Collision and Broadcast
domains!