ccna basic switching and switch configuration

CCNA Guide to Cisco Networking Fundamentals

Chapter 12Basic Switching and Switch

Configuration

CCNA Guide to Cisco Networking Fundamentals 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


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


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


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


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


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


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


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


Latency (continued)


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


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


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


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


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)


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


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


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


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


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


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


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


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


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


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


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


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


Segmentation with Switches (continued)


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


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


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


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 cut-through forwarding– A combination of the cut-through and store-and-

forward methods


Switching Methods (continued)


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


Cut-Through Forwarding (continued)


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


Store-and-Forward Forwarding (continued)


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)


Fragment-Free Forwarding (continued)


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


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


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


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


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


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


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


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


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


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


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


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 basic switching and switch configuration

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