Module 8

1
Module 8

• Ethernet Switching

2
Ethernet Switching

• Ethernet is a shared media
• One node can transmit data at a time
• More nodes increases the demands on the available
  bandwidth
• The probability of collisions increases,
  resulting in more retransmissions
• A solution to the problem is to segment.
• Segmenting creates more collision domains

3
Shared Media Environment

• Shared media environment
• multiple hosts have access to the same medium
• Extended shared media environment
• Using networking devices extends the environment
  to accommodate multiple access or longer cable
  distances
• Point-to-point network environment
• one device is connected to only one other device
  (ex. dialup network connections)

4
Shared media environments
5
Layer 1 Devices

• Layer 1 devices
• repeaters and hubs
• Extend collision domains
• Primary function is extending cable segments
• Additional hosts increase the amount of traffic
• More traffic greater chances of collisions
• This results in diminished performance

6
Repeater Rule

• Four repeater rule
• No more than four repeaters between any two
  computers
• Contributing Factors
• Repeater latency
• Propagation delay
• NIC latency
• Late collision frames add delay that is referred
  to as consumption delay

7
Collision Domains

• Collision Domains
• Connected physical network segments where
  collisions can occur
• Collisions cause
• The network to be inefficient
• Transmissions to stops for a period of time

8
Collision domains
9
Collision Domains

• The types of devices that interconnect the media
  segments define collision domains
• Classified as OSI Layer 1, 2 or 3 devices
• Layer 1 devices do not break up collision domains
• Layer 2 and Layer 3 devices break up collision
  domains
• Increasing the number of collision domains is
  known as segmentation

10
Segmentation
11
Network segment
12
Layer 2 Devices

• Layer 2 devices
• Bridges and Switches
• Segments collision domains
• Controls frame propagation using the MAC address
• Tracks the MAC addresses and segment they are on

13
Layer 2 Bridging
14
Bridges

• Has only two ports and divides a collision domain
  into two parts
• Entire network will share the same logical
  broadcast address space
• Creates more collision domains but will not add
  broadcast domains
• All decisions made are based on MAC or Layer 2
  addressing
• No effect on the logical or Layer 3 addressing

15
Layer 2 Switching
16
Switches

• A switch is a fast, multi-port bridge
• Each port creates its own collision domain
• A switch dynamically builds and maintains a
  Content-Addressable Memory (CAM) table
• The CAM holds all of the necessary MAC
  information for each port

17
Switch Operation

• Micro-segments consist of the switch port and the
  host connected to it
• Communication in both directions at once is known
  as full duplex
• 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.
• A collision domain no longer exists
• Theoretically, the bandwidth is doubled when
  using full duplex

18
(No Transcript)
19
Switch Modes

• Cut-through switching
• A switch transfers the frame as soon as the
  destination MAC address is received
• lowest latency
• no error checking

20
Switch Modes

• Store-and-forward switching
• Higher latency
• The switch receives the entire frame before
  sending it out
• Verifies the Frame Check Sum (FCS)
• Invalid frames are discarded at the switch

21
Switch Modes

• Fragment-free switching
• A compromise between cut-through and
  store-and-forward switching
• Switching begins before the entire data field and
  checksum are read
• Reads the first 64 bytes
• Including the frame header
• Verifies the reliability of
• Addressing
• Logical Link Control (LLC) protocol

22
Switch Modes

• Synchronous switching
• The source port and destination port must be
  operating at the same bit rate
• Asynchronous switching
• 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 must be used  

23
Switch Modes

• Asymmetric switching
• Switched connections between ports of unlike
  bandwidths
• Asymmetric switching is optimized for
  client/server
• A server requires more bandwidth dedicated to the
  server port to prevent a bottleneck at that port

24
Spanning Tree Protocol

• Switching loops can lead to broadcast storms that
  will overwhelm a network.
• To counteract loops, switches are provided with
  the Spanning-Tree Protocol (STP)
• Switches in a LAN using STP
• Send Bridge Protocol Data Units (BPDUs) out all
  its ports
• Lets other switches know of its existence
• Elect a root bridge (switch) for the network
• Switches use the Spanning-Tree Algorithm (STA) to
  resolve and shut down the redundant paths

25
STP

• Each port using Spanning-Tree Protocol is in one
  of the following five states
• Blocking
• Listening
• Learning
• Forwarding
• Disabled

26
STP

• A 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
• Resolving and eliminating loops creates a logical
  hierarchical tree with no loops
• The alternate paths are available if needed

27
Spanning tree protocol
28
Layer 2 Broadcasts

• Ethernet Broadcasts
• When a node needs to communicate with all hosts
  on the network
• A broadcast frame with a destination MAC address
  0xFFFFFFFFFFFF is sent
• The network interface card (NIC) of every host
  must respond

29
Layer 2 Broadcasts

• Layer 2 devices must flood all broadcast and
  multicast traffic
• Broadcast Radiation
• The accumulation of broadcast and multicast
  traffic from each device
• Broadcast storm
• Circulation of broadcast radiation that saturates
  the network
• There is no bandwidth left for application data

30
Layer 2 Broadcasts

• The three sources of broadcasts and multicasts
• Workstations
• Routers
• Multicast Applications

31
Broadcast Collision Domain

• Collision Domain

Collision Domain
32
Layer 3 Devices

• Layer 3 devices
• Routers
• Do not forward collisions
• Breaks up collision domains
• Broadcast domains are controlled

33
Broadcast domain
34
Broadcast Domain

• Broadcast Domain
• A grouping of collision domains
• All the nodes that are a part of that network
  segment bounded by a layer three device
• Broadcasts have to be controlled at Layer 3
  devices
• Layer 2 and Layer 1 devices do not control
  broadcasts

35
Data Flow

• Layer 2 devices filter data frames based on the
  destination MAC address
• A Layer 2 device will forward the frame unless
  something prevents it from doing so
• Layer 3 devices filter data packets based on IP
  destination address
• A Layer 3 device will not forward the frame
  unless it has to
• Layer 3 device creates multiple collision and
  broadcast domains

36
Dataflow
37
Latency

• The delay between the time a frame leaves the
  source device and the time the frame reaches its
  destination
• The following conditions can cause delays
• Physical media
• Circuit delays
• Electronics that process the signal along the
  path
• Software delays
• Decisions that must be made to implement
  switching and protocols
• Delays caused by the content of the frame
• Destination MAC address has to be read

38
Latency