Chapter 13

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CHAPTERCHAPTERTHIRTEENTHIRTEEN

Switching and VLANsSwitching and VLANs

ObjectivesObjectives

• Explain the features and benefits of Fast Ethernet• Describe guidelines and distance limitations of Fast

Ethernet• Define full- and half-duplex Ethernet operations• Distinguish between cut-through, fragment-free, and

store-and-forward LAN switching• Define the operation of the Spanning Tree Protocol and

its benefits• Describe the benefits of virtual LANs• Understand purpose of VLAN trunking protocol (VTP)

Ethernet OperationsEthernet Operations

• Ethernet– A network access method (or media access

method)

– The most pervasive network access method in use– Continues to be the most commonly implemented

media access method in new LANs

CSMA/CDCSMA/CD

• Carrier Sense Multiple Access with Collision Detection (CSMA/CD)– The contention method used by Ethernet

• Interframe gap– Also called interpacket gap (IPG)– Time required between the transmission of data

frames on the network

CollisionsCollisions

• Carrier signal– Transmitted electromagnetic pulse or wave on network

wire indicating transmission is in progress

• Jam signal– 32-bit signal sent by the first station to detect a collision

on an Ethernet network

• Backoff period– Random interval used by devices that have caused a

collision on an Ethernet network during which the devices cannot send

Collision DomainCollision Domain

• Collision domain– The physical area in which a packet collision might

occur

• Repeaters and hubs do not segment the network and therefore do not divide collision domains

• Routers, switches, bridges, and gateways do not segment network and thus create collision domain

LatencyLatency

• Latency– Also called propagation delay– Length of time required to forward, send, or

otherwise or propagate a data frame

• Transmission time– Amount of time it takes for a packet to be sent

from one device and received at another device

LatencyLatency

Table 13-1: Propagation delay for Ethernet media and devices

LatencyLatency

• Slot time– 512 bit times

• 5-4-3 rule– Networking rules that stipulates that between stations on a

10-Mbps half-duplex LAN:• There can be no more than 5 wire segments connected

• Maximum number of repeaters or hubs between the segments is 4

• Maximum number of populated hubs is 3

Ethernet Errors:Ethernet Errors:Frame Size ErrorsFrame Size Errors

• Frame size errors that occur on Ethernet networks:– Short frame

• Also known as runt– Long frame

• Also known as a giant– Jabber

• Longer than Ethernet standards allow and has an incorrect frame check sequence (FCS)

Ethernet Errors:Ethernet Errors:Frame Size ErrorsFrame Size Errors

• Frame check sequence (FCS) error– Indicates that bits of a frame were corrupted during

transmission

– Detected when the calculation at the end of a packet doesn’t conform correctly to the number and sequence of bits in the frame

• If a frame with an FCS error also has an octet missing, it is also called an alignment error

Ethernet Errors:Ethernet Errors:Collision ErrorsCollision Errors

• A large number of devices on a collision domain means a higher chance that there will be a significant number of collisions

• A transmitting station will attempt to send its packet 16 times before discarding it as a NIC error

• Late collision– Occurs when two stations transmit more than 64-bytes

of their data frame before detecting a collision

Ethernet Errors:Ethernet Errors:BroadcastsBroadcasts

• Broadcast– Frame addressed to all stations on broadcast domain

• Broadcast storm– Logical or physical group devices that will receive

broadcast traffic from each other on a LAN

• Protocol analyzer– Can be used to locate the device causing the broadcast

storm

Fast EthernetFast Ethernet

• Defined under the IEEE 802.3u standard

• Has three defined implementations:– 100Base-TX

– 100Base-T4

– 100Base-FX

• Uses multimode fiber-optic (MMF) cable with one TX and one RX strand per link

Half- and Full-Duplex Half- and Full-Duplex CommunicationsCommunications

• Half-duplex– Connection that allows communication in two

directions, but not simultaneously

• Full-duplex– Connection that allows communication in two

directions at once


Figure 13-1: Half-duplex Ethernet communications


• Benefits of using full duplex:– Time is not wasted retransmitting frames because

there are no collisions

– Full bandwidth is available in both directions because the send and receive functions are separate

– Stations do not have to wait to until other stations complete their transmission because there is only one transmitter for each twisted pair


• The four different duplex options:– Auto– Full– Full-flow-control– Half

LAN Segmentation:LAN Segmentation:Segmenting with BridgesSegmenting with Bridges

• Bridge– Segments a network by filtering traffic at the Data

Link layer

• Bridging table– Maintained on the bridge– Maps MAC addresses to the bridge port through

which they can be accessed

When Segmenting a LAN with One or When Segmenting a LAN with One or More Bridges, Note the FollowingMore Bridges, Note the Following

• Bridges reduce collisions by segmenting LAN and filtering 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 individual segment is increased• Bridges can be used to limit traffic for security

purposes by keeping traffic segregated

LAN Segmentation:LAN Segmentation:Segmenting with RoutersSegmenting 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– Typically do not propagate broadcast traffic– Maintain routing tables that include Network layer

addresses of different segments

When Segmenting a LAN with When Segmenting a LAN with Routers, Routers Do the FollowingRouters, Routers Do the Following

• 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

When Segmenting a LAN with When Segmenting a LAN with Routers, Routers Do the FollowingRouters, Routers Do the Following

• 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

• Have a higher latency than bridges because routers have more to process

LAN SwitchingLAN Switching

• Switches– Device that connects devices on a LAN– Segments collision domain by port– 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 SwitchesSegmentation with Switches

• Switches are so similar to bridges, they are often called multiport bridges

• Switches are hardware-controlled• By connecting each port to an individual

workstation, switches microsegment the network

• The bandwidth is not shred as long as each workstation connects to its own port– This is called switched bandwidth


Figure 13-2: Switch packet forwarding actions


Figure 13-3: Packet forwarding decisions made by a switch


• Benefits provided by switches:– Reduction in network traffic and collisions– Increase in available bandwidth per station– Increase in effective distance of a LAN by

dividing it into multiple collision domains– Increased security because unicast traffic is sent

directly to its destination

Switch OperationsSwitch Operations

• Content-addressable memory (CAM)– Memory location on a switch that contains MAC

address-to-switch port mapping information

• A switch uses one of two memory buffers to store frames as it determines to which port a frame will be forwarded– Port-based memory buffering

– Shared memory buffering

Switch OperationsSwitch Operations

• Asymmetric switching– Type of LAN switching that allows for multiple

speeds of network communication

• Symmetric switching– Type of LAN switching that requires all devices to

be operating at the same speed

Securing Switch PortsSecuring Switch Ports

• There are several different degrees of security that can be employed on a switch:– Configure a permanent MAC address for a

specific port on your switch– Define a static a MAC address entry into the

switching table– You can configure port security

Switching MethodsSwitching Methods

Figure 13-4: Catalyst 2820 switching menu

Cut-ThroughCut-Through

• Switching technique in which Ethernet frame is forwarded immediately after destination address is deciphered

• Cisco routers us the term fast forward to indicate that a switch is in cut-through mode

• Any errors occurring in the following fields will not be propagated by the switch:– The preamble– The start frame delimiter (SFD)– The destination address

Cut-ThroughCut-Through

Figure 13-5: Portion of packet read into buffer by a cut-through switch

Store-and-Forward SwitchesStore-and-Forward Switches

• Read the entire into their buffers before forwarding them

Figure 13-6: Entire packet read into buffer by a store-and-forward switch

Fragment-Free SwitchingFragment-Free Switching

• Tries to provide more error-reducing benefits than cut-through switching, while keeping latency lower than store-and-forward switching

• Fragment-free switches are also called modified cut-through switches

13-7: Amount of packet read into buffer by fragment-free switch

Adaptive Cut-Through andAdaptive Cut-Through andChanging the Switch ModeChanging the Switch Mode

• Adaptive cut-through– Also known as error sensing

– Mostly, these switches act like cut-through switches

– If a certain level of errors is detected, switch will change forwarding techniques and act more as store-and-forward switch

• Changing the Switching mode– When you change the switching type, you change it

for all ports on the switch

Spanning Tree ProtocolSpanning Tree Protocol

• Physical path loops– Occur when network devices are connected to one

another by two or more physical media links

• Logical loop– Occurs when a packet can be routed in an endless

loop around a network because bridging tables and/or routing tables reference each other as the destination for a given address

Spanning Tree ProtocolSpanning Tree Protocol

Figure 13-8: Physical loop created on LAN by switches and bridges

Spanning Tree Protocol (STP)Spanning Tree Protocol (STP)

• Invented by Radia Perlman while she was Digital Equipment Corporation (now Compaq) in the 1980s

• Layer 2 link management protocol designed to prevent looping on bridges and switches

• Specification for STP is IEEE802.1d

• Uses the Spanning Tree Algorithm to interrupt the logical loop created by a physical loop in a bridged/switched environment

Building a Logical PathBuilding a Logical Path

• With STP enabled, switches and bridges on a network use an election process to configure a single logical path

• Root bridge– Also called root device– Bridge or switch that is designated the point of

reference in STP operations

Building a Logical PathBuilding a Logical Path

• Bridges use STP to transfer information about each bridge’s MAC address and priority number– The messages the devices send to one another are

called:• Bridge protocol data units (BPDU)

– BPDU messages are sent between root bridge and best ports on other devices, which are called root ports

• Configuration bridge protocol data units (CBPDU)

Port StatesPort States

• The stable states are as follows:– Blocking

– Forwarding

– Disabled

• The transitory states are as follows:– Listening

– Learning

Port StatesPort States

• Ports on STP-enabled devices move through the different states as indicated below:– From bridge/switch bootup or blocking– From blocking to listening (or to disabled)– From listening to learning (or to disabled)– From learning to forwarding (or to disabled)– From forwarding to disabled

Virtual LANVirtual LAN

• Virtual LAN (VLAN)– Grouping of network devices that is not restricted to a

physical segment or switch

• Broadcast domain– Group of network devices that will receive LAN

broadcast traffic from each other

• By default, every port on a switch is in VLAN1– This is the management (or default) VLAN


Figure 13-9: Broadcast domains on a LAN


Figure 13-10: Broadcast domains using VLANs

Benefit of VLANsBenefit of VLANs

• Administrators can divide LANs logically without changing actual physical configuration– This provides administrator with several benefits:

• It is easier to add and move stations on the LAN

• It is easier to reconfigure the LAN

• There is better traffic control

• There is increased security

Benefit of VLANsBenefit of VLANs

Figure 13-11: Securing servers with VLANs

Dynamic Versus Static VLANsDynamic Versus Static VLANs

• Static VLANs are configured port-by-port• In static VLANs, the administrator manually

types in mapping for each port and VLAN• Dynamic VLAN ports can automatically

determine their VLAN configuration• Dynamic VLAN uses a software database of

MAC address-to-VLAN mappings that is created manually

VLAN StandardizationVLAN Standardization

• Frame filtering– Technique used on early VLAN implementations that

employed the use of multiple switching tables

• When creating its VLAN standards, the IEEE did not choose the frame filtering method

• Frame tagging– Also known as frame identification– Method of VLAN identification endorsed by IEEE

802.1q specification

VLAN StandardizationVLAN Standardization

• Two most common types of frame tagging:– 802.1q– Inter-Switch Link (ISL) protocol

• Other types of frame tagging include:– LAN emulation (LANE)– IEEE 802.10 (FDDI)

Creating VLANsCreating VLANs

• Creating VLANs on the Cisco Catalyst 1900 switch using the command line interface is straightforward

• You name the VLANs individually via global configuration mode

• Assigning dynamic VLANs is more complicated than assigning static VLANs, and is not a requirement of CCNA candidates

Link Types and ConfigurationLink Types and Configuration

• There are two types of links on Cisco switches:– Trunk

• Switch-to-switch or switch-to-router links that can carry traffic from multiple VLANs

– Access lists• Links going to non-VLAN-aware devices such as hubs

and individual workstations

Link Types and ConfigurationLink Types and Configuration

• Five different states you can set for a trunk link:– Auto– Desirable– Nonegotiate– Off– On

Trunking ProtocolTrunking Protocol

• VLAN trunking protocol (VTP)– Layer 2 messaging protocol– Manages all changes to the VLANs across

networks

• Any changes made to a VLAN by an administrator are automatically propagated by VTP to all VTP-enabled devices

VTP DomainsVTP Domains

• VTP domain– Group of VTP-enabled devices configured under one

name to share VLAN information

• When you make changes to the VTP configuration, you should verify them with the show vtp command from enable mode

• If all switches are in the same VLAN, there is no need to configure a VTP domain

VTP Device ModesVTP Device Modes

• There are three different modes for VTP-enabled devices:– Server– Client– Transparent

VTP PruningVTP Pruning

• Reduces the number of VTP updates that traverse a link

• Off by default on all switches– If turned on, VTP message broadcasts are only

sent through trunk links that must have the information

• When enabled on a server, it is enabled on every device in the entire domain

Switch Interface DescriptionsSwitch Interface Descriptions

• You can configure a name for each port on a switch

• This is useful when defining roles for a switch port on a global basis– Such as when you configure VLANs

Nonswitching Hubs and VLANsNonswitching Hubs and VLANs

• Considerations to keep in mind when implementing hubs on a network that employs VLANs:– If you insert a hub into a port on a switch and then connect

several devices to the hub, all the system attached to that hub will be in the same VLAN

– If you must move a single workstation that is attached to a hub with several workstations, you will have to physically attach the device to another hub or switch ports in order to change its VLAN assignment

– The more hosts attached at individual switch ports, the greater the microsegmentation and flexibility the VLAN can offer

Routers and VLANsRouters and VLANs

Figure 13-12: Router implemented in a VLAN configuration

Chapter SummaryChapter Summary

• Ethernet (CSMA/CD) is a media access method 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, there will be a collision

• You can segment a network with bridges, switches, or routers to reduce the number of collisions occurring on a network

• Switches do the most to divide collision domains and reduce traffic without dividing broadcast domains


• Switches do the most to divide collision domains and reduce traffic without dividing broadcast domains

• 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 over half-duplex operations

• STP allows administrators to create physical loops between bridges and switches without creating logical loops that would create a problem for packet delivery


• Another way to increase performance, flexibility, and security of a network is to implement VLANs via switches

• VLANs are separate broadcast domains that are not limited by physical configurations

• VLAN information is communicated to switches using the VLAN trunking

Chapter 13

Technology

ethernet errors

ethernet lan

collision errors

ethernet media

ethernet networks

ethernet standards

benefits of fast ethernet

packet collision