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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
2
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
3
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
4
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
5
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
6
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
7
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
8
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
9
Latency (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
10
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
11
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
12
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
13
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
14
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)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
15
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
16
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
17
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
18
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
19
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
20
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
21
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
22
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
23
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
24
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
25
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
26
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
27
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
28
Segmentation with Switches
(continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
29
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
30
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
31
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
32
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
33
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
34
Switching Methods (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
35
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
36
Cut-Through Forwarding (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
37
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
38
Store-and-Forward Forwarding
(continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
39
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)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
40
Fragment-Free Forwarding (continued)
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
41
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
42
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
43
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
44
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
45
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
46
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
47
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
48
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
49
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
50
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
51
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
52
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
CCNA Guide to Cisco Networking Fundamentals, Fourth Edition
53