【文章內(nèi)容簡介】 ?所有站爭用占用媒體時間 ?分布式 (. CSMA/CD) ?實現(xiàn)簡單 ?在負荷適中時有效 ?在重負荷時可能會崩潰 41 MAC Frame Format ?MAC layer receives data from LLC layer ?MAC control ?Destination MAC address ?Source MAC address ?LLC ?CRC ?MAC layer detects errors and discards frames ?LLC optionally retransmits unsuccessful frames 42 MAC幀格式 ?MAC層從 LLC層接收數(shù)據(jù) ?MAC幀的字段 ?MAC控制 ?目的站 MAC地址 ?源站 MAC地址 ?LLC ?CRC (FCS) ?MAC層檢測錯誤并丟棄幀 ?LLC可選設(shè)成重傳不成功接收的幀 43 Typical Frame Format 44 小 結(jié) ?局域網(wǎng)的應(yīng)用 ?拓撲結(jié)構(gòu)和傳輸媒體 ?局域網(wǎng)的體系結(jié)構(gòu) ?IEEE802參考模型 ?媒體接入控制（ Media access control） ?邏輯鏈路控制（ Logical Link Control） 45 Bridges網(wǎng)橋 ?Ability to expand beyond single LAN ?Provide interconnection to other LANs/WANs ?Use Bridge or router ?Bridge is simpler ?Connects similar LANs ?Identical protocols for physical and link layers ?Minimal processing ?Router more general purpose ?Interconnect various LANs and WANs ?see later 46 Why Bridge? ?Reliability ?Performance ?Security ?Geography 47 Functions of a Bridge ?Read all frames transmitted on one LAN and accept those address to any station on the other LAN ?Using MAC protocol for second LAN, retransmit each frame ?Do the same the other way round 48 Bridge Operation 49 Bridge Design Aspects ?No modification to content or format of frame ?No encapsulation ?Exact bitwise copy of frame ?Minimal buffering to meet peak demand ?Contains routing and address intelligence ?Must be able to tell which frames to pass ?May be more than one bridge to cross ?May connect more than two LANs ?Bridging is transparent to stations ?Appears to all stations on multiple LANs as if they are on one single LAN 50 Bridge Protocol Architecture ?IEEE ?MAC level ?Station address is at this level ?Bridge does not need LLC layer ?It is relaying MAC frames ?Can pass frame over external ms system ?. WAN link ?Capture frame ?Encapsulate it ?Forward it across link ?Remove encapsulation and forward over LAN link 51 Connection of Two LANs 52 Fixed Routing ?Complex large LANs need alternative routes ?Load balancing ?Fault tolerance ?Bridge must decide whether to forward frame ?Bridge must decide which LAN to forward frame on ?Routing selected for each sourcedestination pair of LANs ?Done in configuration ?Usually least hop route ?Only changed when topology changes 53 Bridges and LANs with Alternative Routes 102105 101104 or 107？ 54 Spanning Tree支撐樹方法 ?Bridge automatically develops routing table ?Automatically update in response to changes ?3 mechanisms: ?Frame forwarding幀轉(zhuǎn)發(fā) ?Address learning地址探索 ?Loop resolution環(huán)路消除 55 Frame forwarding ?Maintain forwarding database for each port ?List station addresses reached through each port ?For a frame arriving on port X: ?Search forwarding database to see if MAC address is listed for any port except X ?If address not found, forward to all ports except X ?If address listed for port Y, check port Y for blocking or forwarding state ?Blocking prevents port from receiving or transmitting ?If not blocked, transmit frame through port Y 56 Address Learning ?Can preload forwarding database ?Can be learned ?When frame arrives at port X, it has e from the LAN attached to port X ?Use the source address to update forwarding database for port X to include that address ?Timer on each entry in database ?Each time frame arrives, source address checked against forwarding database 57 Spanning Tree Algorithm ?Address learning works for tree layout ?. no closed loops ?For any connected graph there is a spanning tree that maintains connectivity but contains no closed loops ?Each bridge assigned unique identifier ?Exchange between bridges to establish spanning tree 58 Loop of Bridges 59 Layer 2 and Layer 3 Switches ?Now many types of devices for interconnecting LANs ?Beyond bridges and routers ?Layer 2 switches ?Layer 3 switches 60 Hubs集線器 ?