基礎網路管理 第十八章 訊框中繼 製作：林錦財

大綱

本章目標 訊框中繼的目的與範圍 訊框中繼技術 比較點對點與點對多點技術 檢視訊框中繼網路拓樸 配置訊框中繼永久虛擬電路 (PVC) 建立到遠端網路的訊框中繼映射 瞭解非廣播型多重存取網路的課題 瞭解子介面的需求及其配置 驗證與檢修訊框中繼連線

Frame Relay 概念 訊框中繼(Frame Relay)原本為擴充ISDN而 發展。其設計目的在使電路交換技術得以在 分封交換網路上傳輸資料。此已獨立成為一 個建立WAN具成本效益的技術。 較專線便宜 訊框中繼交換機建立虛擬電路以連接多個相 遠的 LAN 到 WAN。Frame Relay 網路介於 LAN 邊緣設備(通常為路由器)與電信業者交 換機之間。

Frame Relay 概述 使用虛擬電路進行連接 提供面向物件的服務 DCE 即 Frame Relay 交換機 CSU/DSU Frame Relay 於此工作 使用虛擬電路進行連接 提供面向物件的服務

EIA/TIA-232, EIA/TIA-449, V.35, X.21, EIA/TIA-530 Frame Relay 堆疊 OSI 參考模型 Frame Relay Application Presentation Session Transport Network IP/IPX/AppleTalk, etc. Data Link Frame Relay EIA/TIA-232, EIA/TIA-449, V.35, X.21, EIA/TIA-530 Physical

Frame Relay 術語 PVC DLCI: 100 DLCI: 200 LMI 100=Active 400=Active Local Access Loop=64 kbps Local Access Loop=T1 PVC Local Access Loop=64 kbps DLCI: 500

虛擬電路 兩部 DTE 之間經由 Frame Relay 網路的連接 稱為虛擬電路 (virtual circuit, VC)。 可以藉由送出訊令到網路上動態建立，這種虛擬 電路稱為可交換的虛擬電路 (switched virtual circuits, SVCs)。並不常用。 通常使用由電信業者事先設定的永久虛擬電路 (permanent virtual circuits, PVCs)。 建立一條 VC 是在每部交換機上儲存輸入埠 (input-port) 到輸出埠 (output-port)的映射 (mapping)，使得由一部交換機鏈結至另一部交換 機，直到識別出一條從一端到另一端的連續路 徑。

DLCI 用以在單一存取鏈路上區別不同的虛擬電路 DLCI 存於每個被傳送訊框的位址欄位 每一條 VC 有自已的資料鏈結通道識別碼 (Data Link Channel Identifier, DLCI) DLCI 存於每個被傳送訊框的位址欄位 DLCI 通常只有局部上的意義 (local significance) 一條VC兩端的 DLCI 可以不相同。 不同的VC可能有一樣的DLCI。

訊框的交換過程

訊框的交換過程

訊框的交換過程

訊框的交換過程

訊框的交換過程

訊框的交換過程

訊框的交換過程

Frame Relay 特性 因其為在高品質數位線路運作而設計，Frame Relay 不提供錯誤復原機制。當任何節點發現 訊框錯誤時，則直接丟棄，並不通知。 FCS 僅在端點計算 Frame Relay 連接設備 (FRAD，即路由器)可 能有多個虛擬電路連到不同的另一端。組態上 每個端點只需要單一的存取鏈路與介面，此相 較於兩兩相連的存取鏈路，具有相當的成本效 益。 另一個節省之處是存取鏈路之容量是基於「平 均頻寬需求」，而非基於「最大頻寬需求」。

Frame Relay 頻寬與流量控制 Frame Relay 為共享網路，但欲達類似電路交 換之服務，需管制頻寬之使用 技術上定義了以下相關術語： 承諾資訊速率(Committed information rate, CIR) 超額資訊速率(Excess information rate, EIR) 適合丟棄的(Discard eligible, DE) 明顯的擁塞通知(Explicit congestion notification, ECN) (參考Cisco CCNA 教材 5.1.4 節 頁籤[4]) 正向(Forward) ECN (FECN) 反向(Backward) ECN (BECN)

節制範例 埠速率：64 Kbps Committed Time (Tc): 0.5 seconds CIR: 12800 bps 計算得 Committed Burst (Bc) = CIR  Tc: 6400 bits 為了方便，假設封包大小正好都是6400 bits 超過，標上DE

節制範例 除了第一個訊框外，全被標上 DE 並且在2.5秒之前不會有不被標上DE的訊框

Frame Relay 位址映射 從提供商那裏得到本地的DLCI號 建立目的地址和本地DLCI之間的映射關係 10.1.1.1 PVC 10.1.1.1 CSU/DSU Inverse ARP 或 Frame Relay map Frame Relay IP (10.1.1.1) DLCI (500) 從提供商那裏得到本地的DLCI號 建立目的地址和本地DLCI之間的映射關係

LMI LMI 擴充包括： 不是資料的DLCI，如右表格： 驗證 VC 正常運作的心跳機制 群播(multicast )機制 流量控制 LMI (ANSI, ITU) 1~15 保留未來使用 992~ 1007 CLLM 1008~ 1022 保留未來使用 (ANSI, ITU) 1019~ 1020 群播(Cisco) 1023 LMI (Cisco)

x Frame Relay 訊令 Cisco 路由器支援三種 LMI 標準: Cisco ANSI T1.617 Annex D DLCI: 500 PVC 10.1.1.1 CSU/DSU x LMI 500=Active 400=Inactive DLCI: 400 PVC Keepalive Cisco 路由器支援三種 LMI 標準: Cisco ANSI T1.617 Annex D ITU-T Q.933 Annex A

Frame Relay 的反轉 ARP 協定和 LMI 工作 1 Frame Relay Cloud DLCI=100 DLCI=400 172.168.5.5 172.168.5.7 Layer 1 of 4: Purpose: This figure describes the Inverse ARP and LMI process. Emphasize: Step 1—Indicates that each router must connect to the Frame Relay switch. Note: The status inquiry messages are part of LMI operation. Explain what Inverse ARP is used for. 參考Cisco CCNA4 5.1.7節 動畫

Frame Relay 的反轉 ARP 協定和 LMI 工作 1 Frame Relay Cloud DLCI=100 DLCI=400 172.168.5.5 172.168.5.7 Status Inquiry Status Inquiry 2 2 Layer 2 of 4: Purpose: This figure describes the Inverse ARP and LMI process. Emphasize: Step 1—Indicates that each router must connect to the Frame Relay switch. Step 2—Discusses what information is sent from the router to the Frame Relay switch.

Frame Relay 的反轉 ARP 協定和 LMI 工作 1 Frame Relay Cloud DLCI=100 DLCI=400 172.168.5.5 172.168.5.7 Status Inquiry Status Inquiry 2 2 Local DLCI 100=Active Local DLCI 400=Active 4 Layer 3 of 4: Purpose: This figure describes the Inverse ARP and LMI process. Emphasize: Step 1—Indicates that each router must connect to the Frame Relay switch. Step 2—Discusses what information is sent from the router to the Frame Relay switch. Step 3—Discusses what the Frame Relay switch does with the received information. 3 3

Frame Relay 的反轉 ARP 協定和 LMI 工作 1 Frame Relay Cloud DLCI=100 DLCI=400 172.168.5.5 172.168.5.7 Status Inquiry Status Inquiry 2 2 Local DLCI 100=Active Local DLCI 400=Active 4 Layer 4 of 4: Purpose: This figure describes the Inverse ARP and LMI process. Emphasize: Step 1—Indicates that each router must connect to the Frame Relay switch. Step 2—Discusses what information is sent from the router to the Frame Relay switch. Step 3—Discusses what the Frame Relay switch does with the received information. Step 4—Discusses the sending of Inverse ARP messages. 3 3 Hello, I am 172.168.5.5. 4

Frame Relay 的反轉 ARP 協定和 LMI 工作 Cloud DLCI=400 DLCI=100 172.168.5.5 172.168.5.7 Frame Relay Map 172.168.5.5 DLCI 400 Active 5 Hello, I am 172.168.5.7. 4 Layer 1 of 3: Purpose: This figure describes the Inverse ARP and LMI process (cont...). Emphasize: Step 5—Discusses how the Inverse ARP message is used to create the Frame Relay map table dynamically. Frame Relay Map 172.168.5.7 DLCI 100 Active 5

Frame Relay 的反轉 ARP 協定和 LMI 工作 Cloud DLCI=400 DLCI=100 172.168.5.5 172.168.5.7 Frame Relay Map 172.168.5.5 DLCI 400 Active 5 Hello, I am 172.168.5.7. 4 Layer 2 of 3: Purpose: This figure describes the Inverse ARP and LMI process (cont...). Emphasize: Step 5—Discusses how the Inverse ARP message is used to create the Frame Relay map table dynamically. Step 6—Shows how Inverse ARP has a periodic interval. Frame Relay Map 172.168.5.7 DLCI 100 Active 5 Hello, I am 172.168.5.5. 6

Frame Relay 的反轉 ARP 協定和 LMI 工作 Cloud DLCI=400 DLCI=100 172.168.5.5 172.168.5.7 Frame Relay Map 172.168.5.5 DLCI 400 Active 5 Hello, I am 172.168.5.7. 4 Layer 3 of 3: Purpose: This figure describes the Inverse ARP and LMI process (cont...). Emphasize: Step 5—Discusses how the Inverse ARP message is used to create the Frame Relay map table dynamically. Step 6—Shows how Inverse ARP has a periodic interval. Step 7—Discusses the periodic interval for keepalive messages. It’s an LMI function. Transition: The next section explains how to configure Frame Relay. Frame Relay Map 172.168.5.7 DLCI 100 Active 5 Hello, I am 172.168.5.5. 6 Keepalives Keepalives 7 7

設定 Frame Relay Rel. 11.2 Router Rel. 10.3 Router HQ Branch interface Serial1 ip address 10.16.0.1 255.255.255.0 encapsulation frame-relay bandwidth 64 interface Serial1 ip address 10.16.0.2 255.255.255.0 encapsulation frame-relay bandwidth 64 frame-relay lmi-type ansi Slide 1 of 2: Purpose: This figure introduces basic Frame Relay configuration over a physical interface. It is important that students understand how configuration occurs in order for them to understand the subinterfaces discussion later in the chapter. These steps assume that LMI and Inverse ARP are supported, therefore no static maps are needed. Regarding step 3: Cisco’s Frame Relay encapsulation uses a 4-byte header, with 2 bytes to identify the DLCI and 2 bytes to identify the packet type. Use the ieft encapsulation to connect to other vendors. The IETF standard is defined in RFCs 1294 and 1490. Regarding step 4: The LMI connection is established by the frame-relay lmi-type [ansi | cisco | q933a] command. The default values established during initial setup are usually sufficient to maintain connectivity with the Frame Relay network. Altering these values would only???These configuration steps are the same, regardless of the network-layer protocols operating across the network. 命令格式 (config-if)# encapsulation frame-relay [cisco | ietf]

設定 Frame Relay Rel. 11.2 Router Rel. 10.3 Router 反轉 ARP 預設情況下是啟動的 HQ Branch interface Serial1 ip address 10.16.0.1 255.255.255.0 encapsulation frame-relay bandwidth 64 interface Serial1 ip address 10.16.0.2 255.255.255.0 encapsulation frame-relay bandwidth 64 frame-relay lmi-type ansi Slide 2 of 2: Purpose: This figure continues the basic Frame Relay configuration over a physical interface. Emphasize: Regarding step 5: This command is used to notify the routing protocol that bandwidth is configured on the link. It is used by IGRP to determine the metric of the link. IGRP uses bandwidth as one of the factors to determine the metric. This command also affects statistics, in particularly statistics in the show interface command. 反轉 ARP 預設情況下是啟動的 在配置輸出資訊中看不出來

設定靜態的 Frame Relay 映射 用於當遠端路由器不支援 Inverse ARP 時 DLCI=110 IP address=10.16.0.1/24 p1r1 HQ Branch DLCI=100 IP address=10.16.0.2/24 Purpose: This figure discusses the static map command option: Emphasize: You can use the frame-relay map command to configure multiple DLCIs to be multiplexed over one physical link. Instead of using Inverse ARP, the Frame Relay map tells the Cisco IOS software how to get from a specific protocol and address pair to ???Point out that this command is similar to building a static route. The simplest way to generate a static map is to let the router learn the information dynamically first. Some users let the router learn the information dynamically, then enable static maps for easier network administration. These configuration steps are the same, regardless of the network-layer protocols operating across the network. Although static maps are not needed when Inverse ARP is enabled, it is a good idea to configure them for each connection for easier network administration. interface Serial1 ip address 10.16.0.1 255.255.255.0 encapsulation frame-relay bandwidth 64 frame-relay map ip 10.16.0.2 110 broadcast 用於當遠端路由器不支援 Inverse ARP 時 或當必須控制PVC上的廣播或群播交通時(用broadcast選項)

查看 Frame Relay 訊息 顯示協定、DLCI、和 LMI 的相關資訊 Router#show interface s0 Serial0 is up, line protocol is up Hardware is HD64570 Internet address is 10.140.1.2/24 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255 Encapsulation FRAME-RELAY, loopback not set, keepalive set (10 sec) LMI enq sent 19, LMI stat recvd 20, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 1023 LMI type is CISCO frame relay DTE FR SVC disabled, LAPF state down Broadcast queue 0/64, broadcasts sent/dropped 8/0, interface broadcasts 5 Last input 00:00:02, output 00:00:02, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops <Output omitted> Slide 1 of 6: Purpose: This figure shows how the show interface command is used to verify whether Frame Relay operation and router connectivity to remote routers are working. Emphasize: Describe the highlighted output to the students. 顯示協定、DLCI、和 LMI 的相關資訊

查看 Frame Relay 訊息 顯示 LMI 資訊 Router#show frame-relay lmi LMI Statistics for interface Serial0 (Frame Relay DTE) LMI TYPE = CISCO Invalid Unnumbered info 0 Invalid Prot Disc 0 Invalid dummy Call Ref 0 Invalid Msg Type 0 Invalid Status Message 0 Invalid Lock Shift 0 Invalid Information ID 0 Invalid Report IE Len 0 Invalid Report Request 0 Invalid Keep IE Len 0 Num Status Enq. Sent 113100 Num Status msgs Rcvd 113100 Num Update Status Rcvd 0 Num Status Timeouts 0 Slide 2 of 6: Purpose: This figure shows how the show frame-relay LMI command is used to verify the LMI type used for signaling. Emphasize: Describe the highlighted output to the students. 顯示 LMI 資訊

查看 Frame Relay 訊息 顯示 PVC 資料通訊資訊 Router#show frame-relay pvc 100 PVC Statistics for interface Serial0 (Frame Relay DTE) DLCI = 100, DLCI USAGE = LOCAL, PVC STATUS = ACTIVE, INTERFACE = Serial0 input pkts 28 output pkts 10 in bytes 8398 out bytes 1198 dropped pkts 0 in FECN pkts 0 in BECN pkts 0 out FECN pkts 0 out BECN pkts 0 in DE pkts 0 out DE pkts 0 out bcast pkts 10 out bcast bytes 1198 pvc create time 00:03:46, last time pvc status changed 00:03:47 Slide 3 of 6: Purpose: This figure shows how the show frame-relay pvc command is used to verify whether Frame Relay operation and router connectivity to remote routers are working. Emphasize: Describe the highlighted output to the students. 顯示 PVC 資料通訊資訊

查看 Frame Relay 訊息 顯示路由資訊 Router#show frame-relay map Serial0 (up): ip 10.140.1.1 dlci 100(0x64,0x1840), dynamic, broadcast,, status defined, active 顯示路由資訊 Slide 4 of 6: Purpose: This figure shows how the show frame-relay map command is used to verify that Frame Relay has a map entry in the Frame Relay map table. Emphasize: Describe the highlighted output to the students.

查看 Frame Relay 訊息 清除動態的Frame Relay 映射關係 Router#show frame-relay map Serial0 (up): ip 10.140.1.1 dlci 100(0x64,0x1840), dynamic, broadcast,, status defined, active Router#clear frame-relay-inarp Router#sh frame map Router# 清除動態的Frame Relay 映射關係 Slide 5 of 6: Purpose: This figure shows how the clear frame-relay-inarp command is used to clear dynamically created Frame Relay maps.

查看 Frame Relay 訊息 顯示 LMI 的調試資訊 Router#debug Frame lmi Frame Relay LMI debugging is on Displaying all Frame Relay LMI data Router# 1w2d: Serial0(out): StEnq, myseq 140, yourseen 139, DTE up 1w2d: datagramstart = 0xE008EC, datagramsize = 13 1w2d: FR encap = 0xFCF10309 1w2d: 00 75 01 01 01 03 02 8C 8B 1w2d: 1w2d: Serial0(in): Status, myseq 140 1w2d: RT IE 1, length 1, type 1 1w2d: KA IE 3, length 2, yourseq 140, myseq 140 1w2d: Serial0(out): StEnq, myseq 141, yourseen 140, DTE up 1w2d: 00 75 01 01 01 03 02 8D 8C 1w2d: Serial0(in): Status, myseq 142 1w2d: RT IE 1, length 1, type 0 1w2d: KA IE 3, length 2, yourseq 142, myseq 142 1w2d: PVC IE 0x7 , length 0x6 , dlci 100, status 0x2 , bw 0 Slide 6 of 6: Purpose: This figure shows how the debug frame-relay lmi command is used to debug your Frame Relay signaling. 顯示 LMI 的調試資訊

選擇 Frame Relay 拓撲結構 Full Mesh Purpose: This figure is a transition discussion to illustrate the need for subinterfaces. Now that students are familiar with the concept and configuring of Frame Relay, they are ready to consider the issues and solutions related to broadcast updates in an???Emphasize: Compare the different topologies described. Explain that by default interfaces that support Frame Relay are multipoint connection types. This type of connection is not a problem when only one PVC is supported by a single interface; but it is when multiple PVCs are supported by a single interface. In???Broadcast routing updates are issued by distance vector protocols. Link-state and hybrid protocols use multicast and unicast addresses. Partial Mesh Star (Hub and Spoke) Frame Relay default: nonbroadcast, multiaccess (NMBA)

路由資訊的可達性 問題: 廣播訊框必須複製給其他已經建立的連接 在NBMA環境的網路中使用水平分割可能造成一些問題 Routing Update 1 B 2 A A C C 3 D Purpose: This figure continues the discussion that leads into the need for subinterfaces. Emphasize: Partial mesh Frame Relay networks must deal with the case of split horizon not allowing routing updates to be retransmitted on the same interface from which they were received. Split horizon cannot be disabled for certain protocols such as Apple???Split horizon issues are overcome through the use of logical subinterfaces assigned to the physical interface connecting to the Frame Relay network. A physical interface can be divided into multiple, logical interfaces. Each logical interface is individually configured and is named after the physical interface. A decimal number is included to distinguish it. The logical port names contain a decimal point and another number indicating these are subinterfaces of interface serial 0 (S0). Subinterfaces are configured by the same configuration commands used on physical interfaces. A broadcast environment can be Frame Relay-created by transmitting the data on each individual circuit. This simulated broadcast requires significant buffering and CPU resources in the transmitting router, and can result in lost user data because of conten???Reference: Interconnections by Radia Perlman is also a good reference on split horizon. Note: Subinterfaces are particularly useful in a Frame Relay partial-mesh NBMA model that uses a distance vector routing protocol. Instead of migrating to a routing protocol that supports turning off split horizon, subinterfaces can be used to overcome the split horizon problem. 問題: 在NBMA環境的網路中使用水平分割可能造成一些問題 廣播訊框必須複製給其他已經建立的連接

實現路由資訊的可達性 Logical Interface Physical Interface Subnet A S0.1 S0.2 S0.3 S0 Subnet B Subnet C 解決方法: 一個解決水平分割問題的方法是使用全連接拓樸 (fully-meshed topology)，但此需要很多 PVC 另一種解決方法是劃分子埠 一個實體埠可以被劃分成多個邏輯意義上的子埠 Purpose: This figure defines subinterfaces and how they can resolve NBMA issues. Emphasize: You can have connectivity problems in a Frame Relay network if these conditions exist: You are using an NBMA model. Your configuration is in a partial mesh. You are using a distance vector routing protocol. Split horizon is enabled on the routing protocol. If the routing protocol is configured with split horizon, routing updates from one router connected on the multipoint subinterface are not propagated to other routers connected on this multipoint subinterface. For example, if router C sends a routing updat???To resolve this problem you can: Use Frame Relay subinterfaces to overcome the split horizon problem. Use a routing protocol that supports disabling split horizon. Use this configuration if you want to save IP address space. You can also use this type of configuration with several fully meshed groups. Routing updates will be exchanged between the fully meshed routers. Note: When an interface is assigned “encapsulation frame-relay,” split horizon is disabled for IP and enabled for IPX and AppleTalk, by default.

子埠的配置 點到點 多點 子埠看作是專線 沒一個點到點連接的子埠要求由自己的子網 適用於星型拓撲結構 子埠應用在 NBMA 網路，因此它們無法解決水平分割所帶來的 問題 由於使用的是單獨的子網可以保存位址空間 適用於 partial-mesh 和 full-mesh 拓撲結構中 Purpose: This figure begins the discussion on configuring subinterfaces. Emphasize: The encapsulation frame-relay command is assigned to the physical interface. All other configuration items, such as the network-layer address and DLCIs, are assigned to the subinterface. Multipoint may not save you addresses if you are using VLSMs. Further, it may not work properly given the broadcast traffic and split horizon considerations. The point-to-point subinterface option was created to avoid these issues. Note: Subinterfaces are also used with ATM networks and IPX LAN environments where multiple encapsulations exist on the same medium.

配置點到點的子埠 frame-relay interface-dlci 命令 用於配置子介面上的本地 DLCI *LMI不懂子介面 10.17.0.1 s0.2 DLCI=110 10.17.0.2 A s0.3 10.18.0.1 DLCI=120 B interface Serial0 no ip address encapsulation frame-relay ! interface Serial0.2 point-to-point ip address 10.17.0.1 255.255.255.0 ?bandwidth 64 frame-relay interface-dlci 110 interface Serial0.3 point-to-point ip address 10.18.0.1 255.255.255.0 frame-relay interface-dlci 120 10.18.0.2 Purpose: This figure continues the discussion of configuring subinterfaces. Emphasize: The Frame Relay service provider will assign the DLCI numbers. These numbers range from 16 to 992. This range will vary depending on the LMI used. Use the frame-relay interface-dlci command on subinterfaces only. Use of the command on an interface, rather than a subinterface, will prevent the device from forwarding packets intended for the DLCI. It is also required for multipoint subinterfaces for wh???Using the frame-relay interface-dlci command with subinterfaces provides greater flexibility when configuring Frame Relay networks. On multipoint subinterfaces, the frame-relay interface-dlci command enables Inverse ARP on the subinterface. When this command is used with point-to-point subinterfaces, all traffic for the subinterface’s subnetwork are sent out this subinterface. The ability to change a subinterface from point-to-point to multipoint, or vice versa, is limited by the software architecture. The router must be rebooted for a change of this type to take effect. An alternative exists to rebooting the router and creating??獸 C frame-relay interface-dlci 命令 用於配置子介面上的本地 DLCI *LMI不懂子介面

多點子埠配置舉例 DLCI=120 s2.2=10.17.0.1/24 s2.1=10.17.0.2/24 DLCI=130 B DLCI=120 s2.2=10.17.0.1/24 s2.1=10.17.0.2/24 DLCI=130 RTR1 RTR3 DLCI=140 s2.1=10.17.0.3/24 interface Serial2 no ip address encapsulation frame-relay ! interface Serial2.2 multipoint ip address 10.17.0.1 255.255.255.0 ?bandwidth 64 frame-relay map ip 10.17.0.2 120 broadcast frame-relay map ip 10.17.0.3 130 broadcast frame-relay map ip 10.17.0.4 140 broadcast RTR4 s2.1=10.17.0.4/24 Purpose: This graphic illustrates a multipoint subinterface example. Emphasize: In this example, the subinterface is configured to behave as a normal NBMA Frame Relay interface. No IP address is configured on the physical interface. It is important that the physical interface NOT have an address, otherwise routing will not work. The frame-relay map command is used to create the multiple PVC connections from a single interface. All connections are in the same subnet. The DLCIs are provided by your service provider.

本章總結 完成本章的學習後，你應該能夠掌握： 瞭解 Frame Relay 工作過程 設定 Frame Relay Purpose: Review the summary items with your students. Emphasize: Read or restate the summary statements. By now, your presentation and classroom discussion should have students able to meet the chapter learning objectives.

問題回顧 1. 什麼是 DLCI? 2. 在Cisco路由器上，用那兩種方法可以建立本地DLCI和目標位址之間的映射關係? 3. 配置Frame Relay的子埠有何好處? Purpose: Review the chapter with open ended questions. Note: The questions in this section are open ended questions designed to foster further discussion. Answers the the review questions are in the “Answers” appendix.