Transcript Chapter 12 - Basic Switching and Switch Configuration

CCNA Guide to Cisco
Networking Fundamentals
Fourth Edition
Chapter 12
Basic Switching and Switch
Configuration
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
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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
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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 rate—100
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
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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
switch’s 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
frame’s 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 cutthrough forwarding
– A combination of the cut-through and store-andforward 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 cutthrough switching
– While keeping latency lower than does store-andforward 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
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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
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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
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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
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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:
– Rm410HL#configure 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:
• Rm410HL#configure terminal
• Rm410HL(config)#interface f0/24
• Rm410HL(config-if)#duplex full
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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 portsecurity ?
– Options include aging, mac-address, maximum,
and violation
• To turn switchport security off, use:
– Rm410HL(config-if)#no switchport portsecurity
• To clear the settings to include erasing the static
MAC addresses, use the clear command:
– Rm410HL(config-if)#clear port-security
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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
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