CCNA 1 Module 8

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CCNA 1 Module 8

• Ethernet Switching

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CCNA 1 Module 8 Objectives

• 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 the Layer 1, 2, and 3 devices used to
  create collision domains and broadcast domains.
• Discuss data flow and problems with broadcasts.
• Explain network segmentation and list the devices
  used to create segments.

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Layer 2 Bridging

• Ethernet is a shared media, meaning only one node
  can transmit data at a time.
• The addition of more nodes increases the demands
  on the available bandwidth and places additional
  loads on the media.
• A solution to the problem is to break the large
  segment into parts and separate it into isolated
  collision domains.
• To accomplish this a bridge keeps a table of MAC
  addresses and the associated ports.
• The bridge then forwards or discards frames based
  on the table entries.

4
Layer 2 Switching

• Generally, a bridge has only two ports and
  divides a collision domain into two parts.
• All decisions made by a bridge are based on MAC
  or Layer 2 addressing and do not affect the
  logical or Layer 3 addressing.
• A bridge will divide a collision domain but has
  no effect on a logical or broadcast domain.
• Unless there is a device such as a router that
  works on Layer 3 addressing, the entire network
  will share the same logical broadcast address
  space.
• A bridge will create more collision domains but
  will not add broadcast domains.
• A switch is essentially a fast, multi-port
  bridge, which can contain dozens of ports. 
• Rather than creating two collision domains, each
  port creates its own collision domain.
• A switch dynamically builds and maintains a
  Content-Addressable Memory (CAM) table, holding
  all of the necessary MAC information for each
  port.

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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.
• These small physical segments are called
  microsegments.
• In a network that uses twisted-pair cabling, one
  pair is used to carry the transmitted signal from
  one node to the other node and a separate pair is
  used for the return or received signal.
• The capability of communication in both
  directions at once is known as full duplex.
• In full duplex mode, there is no contention for
  the media.
• Using CAM allows a switch to directly find the
  port that is associated with a MAC address
  without using search algorithms.
• An application-specific integrated circuit (ASIC)
  is a device consisting of undedicated logic gates
  that can be programmed to perform functions at
  logic speeds.
• The use of these technologies greatly reduced the
  delays caused by software processing and enabled
  a switch to keep pace with the data demands of
  many microsegments and high bit rates.

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Latency

• Latency is the delay between the time a frame
  first starts to leave the source device and the
  time the first part of the frame reaches its
  destination.
• A wide variety of conditions can cause delays as
  a frame travels from source to destination
• Media delays caused by the finite speed that
  signals can travel through the physical media.
• Circuit delays caused by the electronics that
  process the signal along the path.
• Software delays caused by the decisions that
  software must make to implement switching and
  protocols.
• Delays caused by the content of the frame and
  where in the frame switching decisions can be
  made. Example a device cannot route a frame to a
  destination until the destination MAC address has
  been read.

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Switch Modes

• Cut-through switching A switch can start to
  transfer the frame as soon as the destination MAC
  address is received.
• Store-and-forward switching The switch can
  receive the entire frame before sending it out
  the destination port. This gives the switch
  software an opportunity to verify the frame check
  sum (FCS).

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Synchronous and Asynchronous Switching

• When using cut-through methods of switching, both
  the source port and destination port must be
  operating at the same bit rate in order to keep
  the frame intact, known as synchronous switching.
  
• 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, known as asynchronous
  switching.
• Store-and-forward mode must be used for
  asynchronous switching.

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Switch Modes

• A compromise between the cut-through and
  store-and-forward modes is the fragment-free
  mode.
• 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.
• 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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Spanning Tree Protocol

• Loops can occur when extra switches and bridges
  are added to provide redundant paths for
  reliability and fault tolerance.
• A switch sends special messages called bridge
  protocol data units (BPDUs) out all its ports to
  let other switches know of its existence.
• The switches use a spanning-tree algorithm (STA)
  to resolve and shut down the redundant paths
• The protocol used to resolve and eliminate loops
  is known as the Spanning Tree Protocol (STP).

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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
• A port moves through these 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 Media Environments

• It is important to be able to identify a shared
  media environment, because collisions only occur
  in a shared environment.
• Some networks are directly connected and all
  hosts share Layer 1
• Shared media
• Extended shared media
• Point-to-point network

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Shared Media Environments

• Shared media environment multiple hosts have
  access to the same medium.
• Extended shared media environment networking
  devices can extend the environment so that it can
  accommodate multiple access or longer cable
  distances.
• Point-to-point network environment a shared
  networking environment in which one device is
  connected to only one other device, such as
  connecting a computer to an Internet service
  provider by modem and a phone line.

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Collisions and Collision Domains

• Collisions are not inherently bad.
• They are a normal function of Legacy Ethernet.
• Data on the network during a collision is lost
  and usually must be retransmitted.
• Increased collisions indicate congestion.
• All devices on a network that would cause a
  collision if they transmitted simultaneously are
  in a collision domain.
• Networks with only Layer 1 components are a
  single collision domain.

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Shared Media, Repeaters, Hubs, and Collision
Domains
Each is a single collision domain!
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Four-Repeater Rule

• The four repeater rule in Ethernet states that no
  more than four repeaters or repeating hubs can be
  between any two computers on the network.
• Repeater latency, propagation delay, and NIC
  latency all contribute to the four repeater rule.
  
• The 5-4-3-2-1 rule requires that the following
  guidelines should 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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Four-Repeater Rule Example

• The 5-story building shown violates the
  four-repeater rule because host A and B are 5
  repeaters apart.
• Hubs would cause the same result.
• Even if all servers were on the third floor, and
  A and B would never communicate directly they
  are too far to hear each other transmit and can
  cause data collisions.
• What are implications for taller buildings?

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Four-Repeater Rule Example Layer 1 Solution

• The hub added, which could be on any floor,
  allows us to comply with the four-repeater rule.
• No 2 hosts are more than 3 repeaters apart.
• What are implications for taller buildings? It
  really wouldnt matter if each floor connects to
  the hub.
• How many collision domains do we have? Still only
  one and getting bigger with each floor.

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Segmentation

• Layer 2 devices segment or divide collision
  domains.
• Layer 2 devices can control the flow of traffic
  at the Layer 2 level
• allowing data to be transmitted on different
  segments of the LAN at the same time without the
  frames colliding.
• the collision domain is effectively broken up
  into smaller parts, each becoming its own
  collision domain.
• Layer 3 devices, like Layer 2 devices, do not
  forward collisions.
• the use of Layer 3 devices in a network has the
  effect of breaking up collision domains into
  smaller domains.

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Ethernet LAN Segmentation
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Segmenting with Bridges
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Segmenting with Switches
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Segmenting with Routers
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Microsegmentation
Hubs
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Layer 2 Broadcasts

• To communicate with all collision domains,
  protocols use broadcast and multicast frames at
  Layer 2 of the OSI model.
• When a node needs to communicate with all hosts
  on the network, it sends a broadcast frame with a
  destination MAC address 0xFFFFFFFFFFFF, an
  address to which the network interface card (NIC)
  of every host must respond. 
• Layer 2 devices must flood all broadcast and
  multicast traffic, this accumulation of broadcast
  and multicast traffic from each device in the
  network is referred to as broadcast radiation.
• In some cases, the circulation of broadcast
  radiation can saturate the network so that there
  is no bandwidth left for application data.
• In this case, new network connections cannot be
  established, and existing connections may be
  dropped, a situation known as a broadcast storm.

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Broadcast Domains

• A broadcast domain is a grouping of collision
  domains that are connected by Layer 2 devices.
• Broadcasts have to be controlled at Layer 3, as
  Layer 2 and Layer 1 devices have no way of
  controlling them.
• Broadcast domains are controlled at Layer 3
  because routers do not forward broadcasts. 
• In order for a packet to be forwarded through a
  router it must have already been processed by a
  Layer 2 device and the frame information stripped
  off.
• Layer 3 forwarding is based on the destination IP
  address and not the MAC address.
• For a packet to be forwarded it must contain an
  IP address that is outside of the range of
  addresses assigned to the LAN and the 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 in the context of collision and
  broadcast domains focuses on how data frames
  propagate through a network.
• It refers to the movement of data through Layer
  1, 2 and 3 devices and how data must be
  encapsulated to effectively make it from source
  to destination.
• 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 no filtering, so everything
  that is received is passed on to the next
  segment.
• Layer 2 devices filter data frames based on the
  destination MAC address.
• Layer 3 devices filter data packets based on
  destination IP address.
• Layer 1 is used for transmission across the
  physical media, Layer 2 for collision domain
  management, and Layer 3 for broadcast domain
  management.

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Data Flow Through a Network