Link layer, LANs: outline (5C)Ethernet switchSwitch: multiple simultaneous transmissionsSwitch forwarding tableSwitch: self-learningSwitch: frame filtering/forwardingSelf-learning, forwarding: exampleInterconnecting switchesSelf-learning multi-switch exampleInstitutional networkSwitches vs. routersVLANs: motivationVLANsPort-based VLANVLANS spanning multiple switchesPowerPoint PresentationLink layer, LANs: outlineMultiprotocol label switching (MPLS)MPLS capable routersMPLS versus IP pathsSlide 21MPLS signalingMPLS forwarding tablesSlide 24Data center networksSlide 26Slide 27Slide 28Synthesis: a day in the life of a web requestA day in the life: scenarioA day in the life… connecting to the InternetSlide 32A day in the life… ARP (before DNS, before HTTP)A day in the life… using DNSA day in the life…TCP connection carrying HTTPA day in the life… HTTP request/replyChapter 5: SummaryChapter 5: let’s take a breathLink Layer 5-1Link layer, LANs: outline (5C)5.1 introduction, services5.2 error detection, correction 5.3 multiple access protocols5.4 LANsaddressing, ARPEthernetswitchesVLANS5.5 link virtualization: MPLS5.6 data center networking5.7 a day in the life of a web requestLink Layer 5-2Ethernet switchlink-layer device: takes an active rolestore, forward Ethernet framesexamine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segmenttransparenthosts are unaware of presence of switchesplug-and-play, self-learningswitches do not need to be configuredLink Layer 5-3Switch: multiple simultaneous transmissionshosts have dedicated, direct connection to switchswitches buffer packetsEthernet protocol used on each incoming link, but no collisions; full duplexeach link is its own collision domainswitching: A-to-A’ and B-to-B’ can transmit simultaneously, without collisions switch with six interfaces(1,2,3,4,5,6) AA’BB’ CC’123456Link Layer 5-4Switch forwarding tableQ: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?switch with six interfaces(1,2,3,4,5,6) AA’BB’ CC’123456A: each switch has a switch table, each entry:(MAC address of host, interface to reach host, time stamp)looks like a routing table!Q: how are entries created, maintained in switch table? something like a routing protocol?AA’BB’ CC’123456Link Layer 5-5Switch: self-learningswitch learns which hosts can be reached through which interfaceswhen frame received, switch “learns” location of sender: incoming LAN segmentrecords sender/location pair in switch tableA A’Source: ADest: A’MAC addr interface TTLSwitch table (initially empty)A160Link Layer 5-6Switch: frame filtering/forwardingwhen frame received at switch:1. record incoming link, MAC address of sending host2. index switch table using MAC destination address3. if entry found for destination then { if destination on segment from which frame arrived then drop frame else forward frame on interface indicated by entry } else flood /* forward on all interfaces except arriving interface */AA’BB’ CC’123456Link Layer 5-7Self-learning, forwarding: exampleA A’Source: ADest: A’MAC addr interface TTLswitch table (initially empty)A160A A’A A’A A’A A’A A’frame destination, A’, locaton unknown:floodA’ Adestination A location known:A’460 selectively send on just one linkLink Layer 5-8Interconnecting switchesswitches can be connected togetherQ: sending from A to G - how does S1 know to forward frame destined to F via S4 and S3?A: self learning! (works exactly the same as in single-switch case!)ABS1CDEFS2S4S3HIGLink Layer 5-9Self-learning multi-switch exampleSuppose C sends frame to I, I responds to CQ: show switch tables and packet forwarding in S1, S2, S3, S4 ABS1CDEFS2S4S3HIGLink Layer 5-10Institutional networkto externalnetworkrouterIP subnetmail serverweb serverLink Layer 5-11Switches vs. routersboth are store-and-forward: routers: network-layer devices (examine network-layer headers)switches: link-layer devices (examine link-layer headers)both have forwarding tables:routers: compute tables using routing algorithms, IP addressesswitches: learn forwarding table using flooding, learning, MAC addresses applicationtransportnetworklinkphysicalnetworklinkphysicallinkphysicalswitchdatagramapplicationtransportnetworklinkphysicalframeframeframedatagramLink Layer 5-12VLANs: motivationconsider:CS user moves office to EE, but wants connect to CS switch?single broadcast domain:all layer-2 broadcast traffic (ARP, DHCP, unknown location of destination MAC address) must cross entire LAN security/privacy, efficiency issuesComputer ScienceElectricalEngineeringComputerEngineeringLink Layer 5-13VLANsport-based VLAN: switch ports grouped (by switch management software) so that single physical switch ……switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure.Virtual Local Area Network189161027…Electrical Engineering(VLAN ports 1-8)Computer Science(VLAN ports 9-15)15…Electrical Engineering(VLAN ports 1-8)…18279161015…Computer Science(VLAN ports 9-16)… operates as multiple virtual switchesLink Layer 5-14Port-based VLAN189161027…Electrical Engineering(VLAN ports 1-8)Computer Science(VLAN ports 9-15)15…traffic isolation: frames to/from ports 1-8 can only reach ports 1-8can also define VLAN based on MAC addresses of endpoints, rather than switch portdynamic membership: ports can be dynamically assigned among VLANsrouterforwarding between VLANS: done via routing (just as with separate switches)in practice vendors sell combined switches plus routersLink Layer 5-15VLANS spanning multiple switchestrunk port: carries frames between VLANS defined over multiple physical switchesframes forwarded within VLAN between switches can’t be vanilla 802.1 frames (must carry VLAN ID info)802.1q protocol adds/removed additional header fields for frames forwarded between trunk ports1891027…Electrical Engineering(VLAN ports 1-8)Computer Science(VLAN ports 9-15)15…273Ports 2,3,5 belong to EE VLANPorts 4,6,7,8 belong to CS VLAN54 68161Link Layer 5-16type2-byte Tag Protocol Identifier (value: 81-00) Tag Control Information (12 bit VLAN ID field, 3 bit priority field like
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