CCNA Guide to Cisco Networking Fundamentals Fourth Edition

1
CCNA Guide to Cisco Networking Fundamentals
Fourth Edition

• Chapter 12
• Basic Switching and Switch Configuration

2
Objectives

• Explain the technology and media access control
  method for Ethernet networks
• Explain network segmentation and basic traffic
  management concepts
• Explain basic switching concepts and the
  operation of Cisco switches
• Perform and verify switch configuration tasks
• Implement basic switch security

3
Ethernet Operations

• Ethernet
• A network access method (or media access method)
  originated by the University of Hawaii, later
  adopted by Xerox Corporation
• And standardized as IEEE 802.3 in the early 1980s
• Ethernet is
• Most pervasive network access method in use
• Most commonly implemented media access method in
  new LANs

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CSMA/CD

• Carrier Sense Multiple Access with Collision
  Detection (CSMA/CD)
• Ethernet contention method
• Any station connected to a network can transmit
  anytime a transmission is not present on the wire
• Interframe gap, or interpacket gap (IPG)
• After each transmitted signal, each station must
  wait a minimum of 9.6 microseconds before
  transmitting another packet

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CSMA/CD (continued)

• Collisions
• Two stations could listen to the wire
  simultaneously and not sense a carrier signal
• Both stations might begin to transmit their data
  simultaneously
• Once a collision is detected, the first station
  to detect the collision transmits a 32-bit jam
  signal
• Tells all other stations not to transmit for a
  brief period
• The two stations that caused the collision use an
  algorithm to enter a backoff period

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CSMA/CD (continued)

• Collision domain
• The physical area in which a packet collision
  might occur
• Routers, switches, bridges, and gateways segment
  networks
• And thus create separate collision domains
• The 32-bit jam signal that is transmitted when
  the collision is discovered prevents all stations
  on that collision domain from transmitting

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CSMA/CD (continued)

• Broadcasts
• Stations on a network broadcast packets to other
  stations to make their presence known on the
  network
• And to carry out normal network tasks
• When a segment has too much broadcast traffic
• Utilization increases
• Network performance in general suffers
• Simple ways to reduce broadcast traffic
• Reduce the number of services on your network
• Limit the number of protocols in use on your
  network

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CSMA/CD (continued)

• Broadcast storm
• A sudden rush of network transmissions that
  causes all other network communications to slow
  down
• Due to the volume of data competing for access to
  the same bandwidth on the communications medium
• One of the most common causes of broadcast storms
  is a network loop

9
Latency

• Latency, or propagation delay
• The length of time that is required to forward,
  send, or otherwise propagate a data frame
• Latency differs depending on the resistance
  offered by the transmission medium, the number of
  nodes
• And in the case of a connectivity device, the
  amount of processing that must be done on the
  packet
• Transmission time
• The amount of time it takes for a packet to be
  sent from one device to another

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Latency (continued)
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Latency (continued)

• Bit time
• Refers to the amount of time required to transmit
  one data bit on a network
• Slot time (512 bit times)
• An important specification that limits the
  physical size of each Ethernet collision domain
• Specifies that all collisions should be detected
  from anywhere in a network in less time than is
  required to place a 64-byte frame on the network

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Ethernet Errors

• Frame size errors
• Short frame or runt
• Long frame or giant
• Jabber
• Frame check sequence (FCS) error
• Indicates that bits of a frame were corrupted
  during transmission
• Can be caused by any of the previously listed
  errors

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Ethernet Errors (continued)

• Collision errors
• Reducing the number of devices per collision
  domain will usually solve the problem
• You can do this by segmenting your network with a
  router, a bridge, or a switch
• Late collision
• Occurs when two stations transmit more than 64
  bytes of data frames before detecting a collision

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Ethernet Errors (continued)

• Fast Ethernet
• Uses the same CSMA/CD as common 10BaseT Ethernet
• Provides ten times the data transmission rate100
  Mbps
• Defined under the IEEE 802.3u standard
• Implementations
• 100Base-TX
• 100Base-T4
• 100Base-FX

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Gigabit Ethernet

• Recent advances in technology have allowed us to
  reach even higher speeds than those of Fast
  Ethernet
• Gigabit Ethernet implementations
• 1000Base-TX (802.3ab)
• 1000Base-SX (802.3z)
• 1000Base-LX (802.3z)
• 1000Base-CX (802.3z)

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Half- and Full-Duplex Communications

• Half-duplex communications
• Devices can send and receive signals, but not at
  the same time
• Full-duplex (or duplex) communications
• Devices can send and receive signals
  simultaneously
• Ethernet networks can use equipment that supports
  half- and full-duplex communications

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Half- and Full-Duplex Communications (continued)

• Benefits of using full-duplex
• Time is not wasted retransmitting frames because
  collisions do not occur
• The full bandwidth is available in both
  directions because the send and receive functions
  are separate
• Stations do not have to wait until other stations
  complete their transmissions because only one
  transmitter is used for each twisted pair

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Half- and Full-Duplex Communications (continued)

• On a Cisco Catalyst 2950 switch, you can set the
  duplex capabilities port-by-port
• The four different duplex options are
• Auto
• Full
• Full-flow control
• Half

20
A Review of LAN Segmentation

• You can improve the performance of your Ethernet
  network
• By reducing the number of stations per collision
  domain
• Typically, network administrators implement
  bridges, switches, or routers to segment the
  network and divide the collision domains

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Segmenting with Bridges

• Bridge
• Segments a network by filtering traffic at the
  Data Link layer
• Divides a network into two or more segments
• Only forwards a frame from one segment to another
  if the frame is a broadcast or has the MAC
  address of a station on a different segment
• Bridges learn MAC addresses by reading the source
  MAC addresses from frames
• As the frames are passed across the bridge

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Segmenting with Bridges (continued)

• Bridging table
• Maps the MAC addresses on each segment to the
  corresponding port on the bridge to which each
  segment is connected
• Bridges increase latency, but because they
  effectively divide the collision domain
• This does not affect slot time

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Segmenting with Bridges (continued)

• Remember these points
• Bridges reduce collisions on the LAN and filter
  traffic based on MAC addresses
• A bridge does not reduce broadcast or multicast
  traffic
• A bridge can extend the useful distance of the
  Ethernet LAN
• The bandwidth for the new individual segments is
  increased
• Bridges can be used to limit traffic for security
  purposes

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Segmenting with Routers

• Router
• Operates at layer 3 of the OSI reference model
• Interprets the Network layer protocol and makes
  forwarding decisions based on the layer 3 address
• Routers typically do not propagate broadcast
  traffic
• Thus, they reduce network traffic even more than
  bridges do
• Routers maintain routing tables that include the
  Network layer addresses of different segments

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Segmenting with Routers (continued)

• When you segment a LAN with routers, they will
• Decrease collisions by filtering traffic
• Reduce broadcast and multicast traffic by
  blocking or selectively filtering packets
• Support multiple paths and routes between them
• Provide increased bandwidth for the newly created
  segments
• Increase security by preventing packets between
  hosts on one side of the router from propagating
  to the other side of the router

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Segmenting with Routers (continued)

• When you segment a LAN with routers, they will
  (continued)
• Increase the effective distance of the network by
  creating new collision domains
• Provide layer 3 routing, packet fragmentation and
  reassembly, and traffic flow control
• Provide communications between different
  technologies, such as Ethernet and Token Ring or
  Ethernet and Frame Relay
• Have a higher latency than bridges, because
  routers have more to process faster processors
  in the router can reduce some of this latency

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LAN Switching

• Switches
• Similar to bridges in several ways
• Using a switch on a LAN has a different effect on
  the way network traffic is propagated

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Segmentation with Switches

• Switches are often called multiport bridges
• Switch typically connects multiple stations
  individually
• Thereby segmenting a LAN into multiple collision
  domains
• Switches microsegment the network
• By connecting each port to an individual
  workstation
• Switched bandwidth
• Bandwidth is not shared as long as each
  workstation connects to its own switch port

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Segmentation with Switches (continued)
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Segmentation with Switches (continued)

• Switch latency is typically higher than that of a
  repeater or hub
• Faster processors and a variety of switching
  techniques make switches typically faster than
  bridges
• Switches provide the following benefits
• Reduction in network traffic and collisions
• Increase in available bandwidth per station
• Increase in the effective distance of a LAN by
  dividing it into multiple collision domains
• Increased security, because unicast traffic is
  sent directly to its destination

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Switch Operations

• A switch learns the hardware address of devices
  to which it is attached
• By reading the source address of frames as they
  are transmitted across the switch
• The switch then matches the source MAC address
  with the port from which the frame was sent
• The MAC-to-switch-port mapping is stored in the
  switchs content-addressable memory (CAM)
• The switch uses a memory buffer to store frames
  as it determines to which port(s) a frame will be
  forwarded

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Switch Operations (continued)

• Types of memory buffering
• Port-based memory buffering
• Shared memory buffering
• Asymmetric switching
• Some switches can interconnect network interfaces
  of different speeds
• Symmetric switching
• Switches that require all attached network
  interface devices to use the same
  transmit/receive speed

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Switching Methods

• All switches base frame-forwarding decisions on a
  frames destination MAC address
• The three main methods for processing and
  forwarding frames are
• Cut-through, store-and-forward, and fragment-free
• One additional forwarding method, adaptive
  cut-through forwarding
• A combination of the cut-through and
  store-and-forward methods

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Switching Methods (continued)
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Cut-Through Forwarding

• Switches that use cut-through forwarding start
  sending a frame immediately after reading the
  destination MAC address into their buffers
• The main benefit of cut-through forwarding is a
  reduction in latency
• The drawback is the potential for errors in the
  frame that the switch would be unable to detect
• Because the switch only reads a small portion of
  the frame into its buffer

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Cut-Through Forwarding (continued)
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Store-and-Forward Forwarding

• Store-and-forward switches read the entire frame,
  no matter how large, into their buffers before
  forwarding
• Because the switch reads the entire frame, it
  will not forward frames with errors
• The store-and-forward method has the highest
  latency

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Store-and-Forward Forwarding (continued)
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Fragment-Free Forwarding

• Fragment-free forwarding represents an effort to
  provide more error-reducing benefits than
  cut-through switching
• While keeping latency lower than does
  store-and-forward switching
• A fragment-free switch reads the first 64 bytes
  of an Ethernet frame
• And then begins forwarding it to the appropriate
  port(s)

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Fragment-Free Forwarding (continued)
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Fragment-Free Forwarding (continued)

• Adaptive cut-through
• For the most part, the adaptive cut-through
  switch will act as a cut-through switch
• To provide the lowest latency
• However, if a certain level of errors is
  detected, the switch will
• Change forwarding techniques
• Act more as a store-and-forward switch

43
Switch User Interface

• Two types of operating systems are in use on
  Cisco switches IOS-based and set-based
• You can connect to a Cisco switch in the same way
  you connect to a Cisco router
• The Cisco switch has a console port to which you
  can connect your laptop or PC
• Once you power on the switch you will be in the
  command-line interface
• You can configure anything from the command line

44
Modes and Passwords

• You cannot actually configure a switch until you
  get to enable mode
• To enter enable mode, type enable at the
  command-line prompt and then press Enter
• The first step in configuring a switch is to set
  up a password
• To start configuration mode, first type configure
  terminal or config t at the command prompt
• You can also configure a secret (encrypted)
  password

45
Setting the Host Name

• The actual task of setting the host name on the
  Cisco Catalyst switch is identical to setting the
  host name on a Cisco router
• To configure this name, you would type
• Switch(config)hostname name
• Once the host name is set, the prompt will change
  to reflect the name of the switch

46
IP on the Switch

• By default, Cisco switches are not configured
  with IP addresses
• Generally speaking, a switch does not require an
  IP address
• Because switches operate mainly on Layer 2
• You may want to configure an IP address for your
  switch so that you can manage it over the network
  
• Also, you may need to configure an IP address for
  your switch if you want to implement VLANs on
  your network

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Configuring Switch Ports

• To enter interface configuration mode for the
  first port of a switch named Rm410HL, you would
  use the following commands
• Rm410HLconfigure terminal
• Rm410HL(config)interface f0/1
• Rm410HL(config-if)
• To view the configuration of a port, use the show
  command

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Configuring Switch Ports (continued)

• Configuring the duplex mode
• You would use the following command to set the
  duplex mode
• Rm410HLconfigure terminal
• Rm410HL(config)interface f0/24
• Rm410HL(config-if)duplex full

49
Securing Switch Ports

• You can choose from several degrees of security
  on a switch
• First, you can configure a permanent MAC address
  for a specific port on your switch
• Second, you could define a static MAC address
  entry into your switching table
• Which maps a restricted communication path
  between two ports
• To configure port security, you first must enter
  the interface configuration mode

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Securing Switch Ports (continued)

• You can display several options by typing the
  following command
• Rm410HL(config-if)switchport port-security ?
• Options include aging, mac-address, maximum, and
  violation
• To turn switchport security off, use
• Rm410HL(config-if)no switchport port-security
• To clear the settings to include erasing the
  static MAC addresses, use the clear command
• Rm410HL(config-if)clear port-security

51
Summary

• Ethernet (CSMA/CD) is a media access method that
  was developed in the 1960s
• Stations on an Ethernet LAN must listen to the
  network media before transmitting to ensure that
  no other station is currently transmitting
• If two stations transmit simultaneously on the
  same collision domain, a collision will occur
• The transmitting stations must be able to
  recognize the collision and ensure that other
  stations know about it by transmitting a jam
  signal

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Summary (continued)

• The delays caused by collisions on a network can
  seriously affect performance when collisions
  exceed 5 of the traffic on the collision domain
• Switches do the most to divide the collision
  domain and reduce traffic without dividing the
  broadcast domain
• A switch microsegments unicast traffic
• Another way to increase the speed at which a LAN
  operates is to upgrade from Ethernet to Fast
  Ethernet
• Full duplex can also improve Ethernet performance

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Summary (continued)

• Full duplex allows frames to be sent and received
  simultaneously
• As with Fast Ethernet, full-duplex operations are
  only supported by devices designed for this type
  of communication
• The two types of operating systems on Cisco
  switches are IOS-based and set-based
• Configuring a switch is similar to configuring a
  router through the CLI
• Switches can provide some level of security
  through the use of port security commands