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1CCNA 640801 Exam NotesFundamentals of SwitchingCCNA 640801 Exam NotesFundamentals of Switching1LAN Segmentation11 In a collision domain a frame sent by a device can cause collision with a frame sent byanother device in the same collision domainMoreover a device can hear the framesdestined to any device in the same collision domain12 In a broadcast domain a broadcast frame sent by a device can be received by all otherdevices in the same broadcast domain13 A LAN segment or an Ethernet network segment consists of the devices connected witha coaxial cable or a hubThe devices are in the same collision domain14 Ethernet congestion problem occurs when too many devices are connected to the sameEthernet network segment such that the high network bandwidth utilization increases thepossibility of collision which causes degradation of network performance15 LAN segmentation solves the congestion problem by breaking the network into separatesegments or collision domains using bridges switches or routers but not hubs orrepeatersLAN segmentation can reduce the number of collisions in the network andincrease the total bandwidth of the network eg 10 Mbps for one segment 20 Mbps fortwo segments 30 Mbps for three segments and so on16 The 8020 rule should be used when designing how to segment a network ie 80 ormore data traffic should be on the local network segment while 20 or less data trafficshould cross network segments2LAN Switching21 LAN switching or Layer 2 switching consists of three major functions Address learninglearning the MAC addresses of the connected devices to buildthe bridge tableCCNA 640801 Exam NotesFundamentals of Switching2 Forward and filter decisionforwarding and filtering frames based on the bridgetable entries and the bridge logic Loop avoidanceavoiding network loop by using Spanning Tree Protocol22 A bridge or switch maintains a forwarding table also known as bridge table or MACaddress table which maps destination physical addresses with the interfaces or portsto forward frames to the addresses23 A bridge or switch builds a bridge table by learning the MAC addresses of theconnected devicesWhen a bridge is first powered on the bridge table is emptyThebridge listens to the incoming frames and examines the source MAC addresses of theframesFor example if there is an incoming frame with a particular source MACaddress received from a particular interface and the bridge does not have an entry in itstable for the MAC address an entry will be created to associate the MAC address withthe interface24 The default aging time for an entry in a bridge table is 300 seconds 5 minutesItmeans that an entry will be removed from the bridge table if the bridge has not heard anymessage from the concerned host for 5 minutes25 A bridge or switch forwards or filters a frame based on the following logic1 If the destination MAC address of the frame is the broadcast address ieFFFFFFFFFFFF or a multicast address the frame is forwarded out all interfacesexcept the interface at which the frame is received2 If the destination MAC address is an unicast address and there is no associated entryin the bridge table the frame is forwarded out all interfaces except the interface atwhich the frame is received3 If there is an entry for the destination MAC address in the bridge table and theassociated interface is not the interface at which the frame is received the frame isforwarded out that interface only4 Otherwise drop the frame26 There are three types of switching method Storeandforward switching The entire frame is received and the CRC is computed and verified beforeforwarding the frame If the frame is too short ie less than 64 bytes including the CRC too long iemore than 1518 bytes including the CRC or has CRC error it will be discarded It has the lowest error rate but the longest latency for switchingHowever for3CCNA 640801 Exam NotesFundamentals of Switchinghighspeed network eg Fast Ethernet or Gigabit Ethernet network the latencyis not significant It is the most commonly used switching method and is supported by mostswitches Cutthrough switching also known as Fast Forward switching or Real Timeswitching A frame is forwarded as soon as the destination MAC address in the header hasbeen received ie the first 6 bytes following the preamble It has the highest error rate because a frame is forwarded without verifying theCRC and confirming there is no collision but the shortest latency for switching Fragmentfree switching also known as Modified Cutthrough switching A frame is forwarded after the first 64 bytes of the frame have been receivedSince a collision can be detected within the first 64 bytes of a frame collisionwindow size of Ethernet fragmentfree switching can detect a frame corruptedby a collision and drop itTherefore fragmentfree switching provides bettererror checking than cutthrough switching The error rate of fragmentfree switching is above storeandforward switchingand below cutthrough switching The latency of fragmentfree switching is shorter than storeandforwardswitching and longer than cutthrough switching27 Bridges only support storeandforward switchingMost new switch models also usestoreandforward switchingHowever it should be noted that Cisco 1900 switches usefragmentfree switching by default3Spanning Tree31 In a switched network with redundant paths ie with loops the following problems willoccur Broadcast StormA broadcast or multicast frame will be forwarded by a switch outall its active ports except the source portThe resulted frames will then beforwarded by the other switches in the network similarlySome of the frames willbe forwarded around the network loop and back to the original switchThe processthen repeatsTherefore the frames will loop indefinitely in the network andeventually exhaust the processing power of the switches and the bandwidth of thenetwork Receiving multiple copies of a frameWhen a switch receives an unicast frame toa destination device that it does not have an entry in its bridge table it will forward
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