CCNA Guide to Cisco Networking Fundamentals Fourth Edition

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 1980 s • 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 10 Base. T Ethernet – Provides ten times the data transmission rate— 100 Mbps – Defined under the IEEE 802. 3 u standard • Implementations – 100 Base-TX – 100 Base-T 4 – 100 Base-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 – – 1000 Base-TX (802. 3 ab) 1000 Base-SX (802. 3 z) 1000 Base-LX (802. 3 z) 1000 Base-CX (802. 3 z) 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

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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 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-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

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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 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-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-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 • 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 Rm 410 HL, you would use the following commands: – Rm 410 HL#configure terminal – Rm 410 HL(config)#interface f 0/1 – Rm 410 HL(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: • Rm 410 HL#configure terminal • Rm 410 HL(config)#interface f 0/24 • Rm 410 HL(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: – Rm 410 HL(config-if)#switchportsecurity ? – Options include aging, mac-address, maximum, and violation • To turn switchport security off, use: – Rm 410 HL(config-if)#no switchportsecurity • To clear the settings to include erasing the static MAC addresses, use the clear command: – Rm 410 HL(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 1960 s • 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