計(jì)算機(jī)網(wǎng)絡(luò)工程-ch4(6學(xué)時(shí)(文件)
【正文】 R3 R2 R1 網(wǎng) 8 網(wǎng) 6 網(wǎng) 3 網(wǎng) 2 網(wǎng) 1 網(wǎng) 7 區(qū)域 網(wǎng) 4 網(wǎng) 5 R8 2020/6/16 Nanjing university of industry WuJun 111 劃分區(qū)域 ? 劃分區(qū)域的好處就是將利用洪泛法交換鏈路狀態(tài)信息的范圍局限于每一個(gè)區(qū)域而不是整個(gè)的自治系統(tǒng)，這就減少了整個(gè)網(wǎng)絡(luò)上的通信量。主干區(qū)域的標(biāo)識(shí)符規(guī)定為 。 SPF算法將每一個(gè)路由器作為根來(lái)計(jì)算其到每一個(gè)目的地路由器的距離，每一個(gè)路由器根據(jù)一個(gè)。 2020/6/16 Nanjing university of industry WuJun 112 自治系統(tǒng) AS 主干路由器 區(qū)域 區(qū)域 主干區(qū)域 至其他自治系統(tǒng) R9 R7 R6 R5 R4 R3 R2 R1 網(wǎng) 8 網(wǎng) 6 網(wǎng) 3 網(wǎng) 2 網(wǎng) 1 網(wǎng) 7 區(qū)域 網(wǎng) 4 網(wǎng) 5 R8 2020/6/16 Nanjing university of industry WuJun 113 自治系統(tǒng) AS 區(qū)域邊界路由器 區(qū)域 區(qū)域 主干區(qū)域 至其他自治系統(tǒng) R9 R7 R6 R5 R4 R3 R2 R1 網(wǎng) 8 網(wǎng) 6 網(wǎng) 3 網(wǎng) 2 網(wǎng) 1 網(wǎng) 7 區(qū)域 網(wǎng) 4 網(wǎng) 5 R8 LinkState Routing Protocols ? After initial flood, pass small eventtriggered linkstate updates to all other routers LinkState Packets SPF Algorithm Topological Database Shortest Path First Tree Routing Table C A D B 2020/6/16 Nanjing university of industry WuJun 115 SPF算法是 OSPF路由協(xié)議的基礎(chǔ)。 ? OSPF 使用層次結(jié)構(gòu)的區(qū)域劃分。 ? 每一個(gè)區(qū)域都有一個(gè) 32 bit 的區(qū)域標(biāo)識(shí)符（用點(diǎn)分十進(jìn)制表示）。 ? 這個(gè)數(shù)據(jù)庫(kù)實(shí)際上就是全網(wǎng)的拓?fù)浣Y(jié)構(gòu)圖，它在全網(wǎng)范圍內(nèi)是一致的（這稱(chēng)為鏈路狀態(tài)數(shù)據(jù)庫(kù)的同步）。 ? 發(fā)送的信息就是與本路由器相鄰的所有路由器的鏈路狀態(tài)，但這只是路由器所知道的部分信息。 2020/6/16 Nanjing university of industry WuJun 106 內(nèi)部網(wǎng)關(guān)協(xié)議 OSPF (Open Shortest Path First) OSPF 協(xié)議的基本特點(diǎn) ? “開(kāi)放”表明 OSPF 協(xié)議不是受某一家廠商控制，而是公開(kāi)發(fā)表的。 2020/6/16 Nanjing university of industry WuJun 105 RIP 協(xié)議的優(yōu)缺點(diǎn) ? RIP 存在的一個(gè)問(wèn)題是當(dāng)網(wǎng)絡(luò)出現(xiàn)故障時(shí) ， 要經(jīng)過(guò)比較長(zhǎng)的時(shí)間才能將此信息傳送到所有的路由器 。 每個(gè)路由信息需要用 20 個(gè)字節(jié) 。 消除路由循環(huán)的基本方法： 2020/6/16 Nanjing university of industry WuJun 102 RIP 協(xié)議的位置 ? RIP 協(xié)議使用運(yùn)輸層的用戶(hù)數(shù)據(jù)報(bào) UDP進(jìn)行傳送 （ 使用 UDP 的端口 520） 。 Distance Vector Routing Protocols ? Pass periodic copies of routing table to neighbor routers and accumulate distance vectors C D B A C B A D Routing Table Routing Table Routing Table Routing Table Distance— How far Vector— In which direction ? Routers discover the best path to destinations from each neighbor A B C E0 S0 S0 S1 S0 E0 Routing Table 0 0 S0 S1 Routing Table S0 0 E0 0 Routing Table E0 S0 0 0 Distance Vector— Sources of Information and Discovering Routes ? Routers discover the best path to destinations from each neighbor A B C E0 S0 S0 S1 S0 E0 Routing Table Routing Table 0 0 1 1 S0 S1 S1 S0 Routing Table S0 0 E0 0 S0 1 E0 S0 S0 1 0 0 Distance Vector— Sources of Information and Discovering Routes Distance Vector— Sources of Information and Discovering Routes ? Routers discover the best path to destinations from each neighbor A B C E0 S0 S0 S1 S0 E0 Routing Table Routing Table 0 0 1 1 S0 S1 S1 S0 Routing Table S0 0 E0 0 S0 S0 1 2 E0 S0 S0 S0 1 2 0 0 Distance Vector— Maintaining Routing Information ? Updates proceed stepbystep from router to router A Process to update this routing table Topology change causes routing table update Distance Vector— Maintaining Routing Information ? Updates proceed stepbystep from router to router A Process to update this routing table Router A sends out this updated routing table after the next period expires Topology change causes routing table update Distance Vector— Maintaining Routing Information ? Updates proceed stepbystep from router to router A B Process to update this routing table Process to update this routing table Topology change causes routing table update Router A sends out this updated routing table after the next period expires Maintaining Routing Information Problem— Routing Loops ? Each node maintains the distance from itself to each possible destination work A B C E0 S0 S0 S1 S0 E0 Routing Table S0 E0 S0 S0 1 2 0 0 Routing Table E0 S0 S0 S0 1 2 0 0 Routing Table S0 S1 S1 S0 1 1 0 0 Maintaining Routing Information Problem— Routing Loops ? Slow convergence produces inconsistent routing A B C E0 S0 S0 S1 S0 E0 X Routing Table S0 E0 S0 S0 1 2 0 Down Routing Table E0 S0 S0 S0 1 2 0 0 Routing Table S0 S1 S1 S0 1 1 0 0 Router C concludes that the best path to work is through Router B Maintaining Routing Information Problem— Routing Loops A B C E0 S0 S0 S1 S0 E0 X Routing Table S0 S0 S0 S0 1 2 0 2 Routing Table E0 S0 S0 S0 1 2 0 0 Routing Table S0 S1 S1 S1 1 1 0 0 Router A updates its table to reflect the new but erroneous hop count Maintaining Routing Information Problem— Routing Loops A B C E0 S0 S0 S1 S0 E0 X Routing Table S0 S0 S0 S0 1 2 0 2 Routing Table E0 S0 S0 S0 1 4 0 0 Routing Table S0 S1 S1 S0 3 1 0 0 Symptom: Counting to Infinity ? Packets for work bounce between routers A, B, and C ? Hop count for work counts to infinity A B C E0 S0 S0 S1 S0 E0 X Routing Table S0 S0 S0 S0 1 2 0 4 Routing Table E0 S0 S0 S0 1 6 0 0 Routing Table S0 S1 S1 S0 5 1 0 0 Solution: Defining a Maximum ? Define a limit on the number of hops to prevent infinite loops A B C E0 S0 S0 S1 S0 E0 X Routing Table S0 S0 S0 S0 1 2 0 16 Routing Table E0 S0 S0 S0 1 16 0 0 Routing Table S0 S1 S1 S0 16 1 0 0 Solution 1: Split Horizon ? It is never useful to send informa
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