Tài liệu WAN Technologies pdf

1 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ 9 WAN Technologies Terms you’ll need to understand: ✓ Integrated Services Digital Network (ISDN) ✓ Point-to-Point Protocol (PPP) ✓ Dial on Demand Routing (DDR) ✓ High-Level Data Link Control (HDLC) ✓ X.25 ✓ Frame Relay ✓ Leased lines ✓ Asynchronous Transfer Mode (ATM) Techniques you’ll need to master: ✓ Differentiating among wide area network services, including ISDN, X.25, Frame Relay, ATM, and leased-line protocols ✓ Recognizing key WAN terms for ISDN, Frame Relay, X.25, ATM, and leased-line protocols ✓ Using common commands to view the status of WAN links 2 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Chapter 9 This chapter examines the methods used to transport data over wide area networks. We will cover common WAN technologies, such as Frame Relay and ATM, and how they are configured on Cisco routers. Also, this chapter covers the following WAN-related CCIE blueprint objectives, as laid out by the Cisco Systems CCIE program: ➤ Integrated Services Digital Network (ISDN)—Link Access Procedure on the D channel (LAPD), Basic Rate Interface (BRI) framing, Primary Rate Interface (PRI) framing, signaling, mapping, dialer map, interface types, B/D channels, PPP Multilink ➤ X.25—Addressing, routing, Link Access Procedure Balanced (LAPB), error control/recovery, windowing, signaling, mapping, Switched Virtual Circuit (SVC)/Permanent Virtual Circuit (PVC), Protocol Translation ➤ Frame Relay—Local Management Interface (LMI), Data Link Connection Identifier (DLCI), permanent virtual circuit (PVC), framing, traffic shaping, Forward Explicit Congestion Notification (FECN), Backward Explicit Congestion Notification (BECN), Committed Information Rate (CIR), Discard Eligibility (DE), mapping, compression ➤ Asynchronous Transfer Mode (ATM)—SVC/PVC, ATM Adaptation Layer (AAL), Service Specific Connection Oriented Protocol (SSCOP), User- Network Interface (UNI), Network-Network Interface (NNI), Interim Local Management Interface (ILMI), cell format, quality of service (QoS), RFC 1483, RFC 1577, Private Network-Network Interface (PNNI), Interim- Interswitch Signaling Protocol (IISP), mapping ➤ Leased-line protocols—High-Level Data Link Control (HDLC), Point-To- Point Protocol (PPP), async, modems, compression ➤ Dial on Demand Routing (DDR)—Short term WAN connections, dial backup ➤ Physical layer—Synchronization, Synchronous Optical Network (SONET), T1, E1, encoding As with other chapters in this book, additional information is provided for complete- ness and in preparation for additional subjects as the CCIE program expands. WAN Technologies Wide area networks (WANs) provide connectivity across longer distances than local area networks (LANs), and WANs are typically slower than LANs because they provide connectivity over wider geographical distances. This chapter focuses on the following topics: 3 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ WAN Technologies ➤ ISDN ➤ PPP ➤ X.25 ➤ Frame Relay ➤ ATM Due the to high cost of WANs, Cisco introduced Dial on Demand Routing (DDR) to provide a WAN connectivity only when end user data is sent across to a remote or central location. DDR is used to reduce the ongoing costs of having a dedicated WAN circuit active all the time. We will begin our discussion of WAN technologies by exploring Integrated Ser- vices Digital Network (ISDN) first. ISDN Integrated Services Digital Network (ISDN) is a digital service that enables network users to send and receive data, voice, and video transmissions over a network. ISDN offers a variety of link speeds, ranging from 64Kbps to 2.048Mbps. Therefore, many small- and medium-sized companies find that ISDN is a viable network solution. Basic Rate and Primary Rate Interfaces ISDN can be supplied by a carrier in two main forms—Basic Rate Interface (BRI) and Primary Rate Interface (PRI). An ISDN BRI consists of two 64Kbps services (B channels) and one 16Kbps signaling channel (D channel). An ISDN PRI consists of 23 B or 30 B channels, depending on the country. In North America and Japan, a PRI service consists of 23 B channels. In Europe and Aus- tralia, a PRI service consists of 30 B channels. A signaling channel (or D chan- nel) is used in a PRI service and is a dedicated 64Kbps channel. Note: The effective throughput of a PRI service with 23 channels is 1.472Mbps (23x64Kbps). With 30 B channels, the effective throughput is 1.920Mbps (30x64Kbps). The International Telecommunications Union (ITU) defines the standards for ISDN. ISDN Framing and Frame Format The ISDN Physical layer provides the ability to send outbound traffic and receive inbound traffic by transmitting binary bits over the physical media. The ISDN Data Link layer provides signaling, which ensures that data is sent and received correctly. 4 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Chapter 9 The signaling protocol used in ISDN is called the Link Access Procedure on the D channel (LAPD). Figure 9.1 shows LAPD’s frame format. The flag field in an LAPD frame (see Figure 9.1) is used to delimit the frame. The frame control status field is a calculation used to determine whether any errors have occurred during a frame’s delivery. If the error-checking calculation result differs from the value in the original frame, an error is generated by the network or router and sent to higher layers for retransmission. Note: The LAPD frame format is similar to the High-Level Data Link Control (HDLC), which is discussed later in this chapter. HDLC is used for point-to-point connections. Layer 3, the Network layer, of the ISDN model is used to provide call establishment, call termination, and information transfer. Before we look at an example of configuring ISDN on a Cisco router using a simple network design, you need to understand leased line protocols so you can apply this knowledge to complex scenarios, such as authentication techniques in Point-to-Point protocols (PPP). Leased Line Protocols A leased line is a service provided by a carrier that maintains a connection between two remote networks separated by some geographical region. These remote sites can range from SOHO (small office home office) to satellite corporate offices. HDLC High-Level Data Link Control (HDLC) is a WAN protocol encapsulation method that allows point-to-point connections between two remote sites. Typically, 1 Flag 2 Address 1 Control 1 FCS 1 Flag Variable Length Data SAPI C/R EA TEI EA SAPI – Service Access Point Identifier C/R – Command/Response EA – Extended Access TEI – Terminal Endpoint Identifier (all ones indicate a broadcast) Field length in bytes Figure 9.1 LAPD frame format. 5 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ WAN Technologies HDLC is used in a leased-line setup. HDLC is a connectionless protocol that relies on upper layers to recover any frames that have encountered errors across a WAN link. Because HDLC is proprietary, you cannot use HDLC between a Cisco router and another vendor. HDLC is the default encapsulation on Cisco serial interfaces. Cisco routers use HDLC encapsulation, which is proprietary. This proprietary nature is due to the fact that Cisco added an address field in the HDLC frame, which is not present in the HDLC standard, this field is used by Cisco devices to indicate the type of payload (protocol). Cisco routers use the address field in an HDLC frame to indicate a payload type, but other routers or manufacturers that implement the HDLC standard do not use the address field. Point-to-Point Protocol (PPP) PPP was designed to transport user information between two WAN devices (also referred to as point-to-point links). PPP was designed as an improvement over Serial Line Internet Protocol (SLIP). SLIP provided basic IP connectivity. When PPP encapsulation is configured on a Cisco WAN interface, the network administrator can carry protocols such as IP and IPX as well as many others. Cisco routers support PPP over asynchronous lines, High-Speed Serial Interfaces (HSSI), ISDN lines, and synchronous serial ports. PPP has the added function of allowing authentication to take place before any end user data is sent across the link. The following three phases occur in any PPP session: 1. Link Establishment—Link Control Program (LCP) packets are sent to con- figure and test the link. 2. Authentication (optional)—After the link is established, authentication can be used to ensure that link security is maintained. 3. Network layers—In this phase; Network Control Program (NCP) packets deter- mine which protocols will be used across the PPP link. An interesting aspect of PPP is that each protocol (IP, IPX, and so on) supported in this phase is documented in a separate RFC that discusses how it operates over PPP. LCP Link Control Protocol (LCP) is used to establish, configure, and test the link between two devices, such as Cisco routers. LCP provides the necessary negotiations between end devices to activate the link. Once the link is activated, but no data is flowing, the next phase of the PPP session can take place, authentication (if configured) and the NCP. 6 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Chapter 9 Authentication PPP supports authentication via Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP), with CHAP providing a more secure method of authentication. CHAP passwords are encrypted and safe from intruders because they are never actually transmitted on the wire. This technique is known as shared secrets in that both devices know the secret (password), but they never talk about it directly. PAP passwords are sent in clear text, which means they are clearly visible on the wire. NCP PPP uses Network Control Program (NCP) packets to allow multiple protocol types to transfer across WANs from point to point. IP Control Program (IPCP) allows IP connectivity, and IPXCP allows IPX connectivity. Figure 9.2 displays the PPP model compared to the OSI model. PPP consists of three main components: ➤ High-Level Data Link Control (HDLC)—Provides for encapsulating datagrams over PPP links. ➤ Link Control Protocol (LCP)—Establishes, configures, and tests a PPP link. ➤ Network Control Program (NCP)—Configures many different network layer protocols. Now that we’ve reviewed PPP basics, let’s configure a simple network imple- mentation using PPP across an ISDN link. Application Presentation Session Transport Network Data Link Physical OSI Model PPP IPCP, IPXCP Network Control Protocol (NCP) Link Control Protocol (LCP) Physical PPP Model Figure 9.2 The PPP model. 7 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ WAN Technologies Configuring ISDN on a Cisco Router Configuring ISDN on a Cisco router requires two main steps: 1. Identify the ISDN switch type in global configuration mode. 2. Configure the desired interface parameters. Figure 9.3 shows two Cisco routers connected via an ISDN BRI service. On Cisco routers, a BRI service is indicated by the interface name of BRI. For PRI services, the router identifies the service with either T1 (23 B channels) or E1 (For 30 B channels). In this example, router R1 will be configured using the BRI port on the Cisco router to send user data as required across one B channel, and the ISDN switch type is basic-net3. We will also be using CHAP authentication. Listing 9.1 (which is truncated) displays the configuration for router R1. Note: The BRI interface name is a combination of BRI along with the BRI interface number (for example, BRI1, BRI2, and so on). Listing 9.1 The show running-confiig command on R1. R1#show running-config version 12.0 hostname R1 enable password 7 1511021F0725 username R2 password cisco ! Define the isdn switch type with the following command isdn switch-type basic-net3 interface Ethernet0 ip address 10.1.1.1 255.255.255.0 ! BRI interfaces are names BRI on a Cisco router interface BRI0 ip address 131.108.1.1 255.255.255.0 !We are using PPP and PPP authentication encapsulation PPP PPP authentication chap dialer-group 1 dialer map ip 131.108.1.2 name R2 broadcast 0293353020 ! router rip network 131.108.0.0 ! Permit all IP traffic across B channel dialer-list 1 protocol ip permit ! line con 0 line vty 0 4 login 8 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Chapter 9 In the configuration shown in Listing 9.1, the Point-to-Point Protocol is used to run over the ISDN B-channel. The dialer-group 1 command is associated with the dialer-list command, which identifies that all IP traffic across the link. This is also known as defining interesting traffic. Interesting traffic is data that is important enough to the end user to warrant bringing up the WAN connection. The IOS command dialer map ip maps the next hop address to router R2 to the Sydney, Australia, ISDN phone number 0293353020. The router in Listing 9.1 is configured with the broad- cast routing protocol IP RIP. IP RIP sends all updates as IP broadcasts; hence, the broadcast keyword used in the dialer map statement. Using IP RIP or any other broadcast protocol ensures that the BRI link always remains active. Only IP based traffic will activate the link, and while the link is active, any other protocol may be carried across the WAN. You use the dialer-list command to advise the router what protocol, can activate the link. For example, if you wanted to enable IPX to activate the link (or interesting traffic), you would have to add the following command: dialer-list 1 protocol ipx permit Listing 9.2 displays a successful ping from router R1 to router R2 after PPP has been configured. Listing 9.2 The ping command on router R1. R1#ping 131.108.1.2 Type escape sequence to abort. Sending 5,100-byte ICMP Echos to 131.108.1.2,timeout is 2 seconds: !!!!! Success rate is 100 percent(5/5),round-trip min/avg/max=36/36/40ms R1# ISDN service provider switch type is basic-net3 BRI0 131.108.1.1/24 BRI0 131.108.1.2/24 R1 R2 ISDN Number 0293353020 Figure 9.3 ISDN configuration example. 9 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ WAN Technologies Table 9.1 displays some useful ISDN-related show and debug IOS commands. PPP multilink is simply the ability to add more B channels together so that bandwidth is increased from 64Kbps up to 30 B channels or 1.920Mbps. Now that we have covered ISDN, let’s move onto a protocol that will enable the least expensive method of allowing communication between two remote sites, namely Dial on Demand Routing (DDR). Dial on Demand Routing (DDR) Dial on Demand Routing (DDR) enables routing information to be initiated and closed as required by transmitting stations, such as PCs. A DDR link is only operational when information exchange is required. Typically, DDR is deployed as a backup connectivity solution in case the primary WAN link goes down. DDR is used in environments with a low volume of traffic over an ISDN or Public Switched Telephone Network (PSTN). DDR can also be used as a backup to a dedicated leased-line service. Figure 9.4 displays a typical situation in which DDR and dial backup might be used. Figure 9.4 displays two Cisco routers (named Melanie and Ben) connected over an ISDN or PSTN line (ISDN would require a BRI interface on the Cisco router, and PSTN would require an external modem). The routers, Melanie or Ben, have only low volume traffic to send. DDR can be used to maintain a link between to two routers when required. This setup results in financial savings over a long period of time. In general, the process of transferring data between two routers using DDR goes like this: 1. Traffic defined as interesting arrives at the router forcing the router to acti- vate the dial up interface in order to transmit the data properly. Table 9.1 ISDN show and debug commands. IOS Command Description show isdn interface Displays information on a specific B channel. show isdn Displays information about memory, layer 2, and layer 3 timers. debug bri Displays information about ISDN BRI routing activity. debug isdn events Displays information about user events that occur on the interface. debug isdn q921 Displays layer 2 access procedures. debug isdn q931 Displays layer 3 information about call setup and call termination. 10 ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ Chapter 9 2. Before data can be transferred between the two routers, the routers must activate the WAN connection and exchange routing information. 3. After routing information is exchanged, data can be transferred. 4. After the data transfer is complete, a configurable timeout option expires, and the link disconnects. As mentioned earlier, DDR can also be used to provide a backup option if a main leased line goes down due to a carrier or router failure. Figure 9.4 displays a network that contains a primary link and a backup ISDN link. Note: Remember, an ISDN service is tariffed according to usage, so ISDN service is typically only billed when active. For illustrative purposes, let’s assume the leased line between the Melanie and Ben routers has failed. Either the Melanie or the Ben routers can bring up the ISDN service to maintain connectivity. When the leased line becomes active again, the ISDN line can be brought down. Listing 9.3 displays a sample configuration on the Melanie router to enable using dial backup. Listing 9.3 Sample DDR backup configuration. hostname Melanie . interface S0 ip address 1.1.1.1 255.255.255.0 backup interface bri0 backup delay 0 120 backup load 80 50 In the event that the link between the Ben and Melanie routers shown in Figure 9.4 fails, the configuration shown in Listing 9.3 will activate the backup link and ensure network connectivity between the two sites. ISDN or PSTN Melanie Ben Low volume data traffic to Router Ben Low volume data traffic to Router Melanie Figure 9.4 Typical DDR and dial backup application. [...]... data must be sent on average every 90 seconds ❍ c When data must be sent as required but the WAN link remains idle when no data is sent ❍ d Can only be used with IP data The correct answer is c Dial on Demand Routing (DDR) is used to send data over a WAN line only when necessary When no data is sent across the WAN link (or it is idle), the circuit is closed to save on usage costs Answer a is incorrect,... and before activating the backup connection and then to wait 120 seconds after the main WAN link has been restored before tearing down the backup link Another useful purpose of DDR is to provide extra bandwidth for an existing circuit in periods of high demand The backup load 80 50 IOS command brings up another WAN circuit if the load on an exiting line reaches 80 percent and brings down the backup... Figure 9.6, the layer 3 protocol called Packet-Layer Protocol (PLP) provides addressing for X.25 devices, and the Physical layer is concerned with how bits are transferred across the physical wire WAN Technologies 13 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ For example, X.21bis is a physical media specification that provides the electrical and mechanical properties... across an X.25 link R1#ping 131.108.3.2 Type escape sequence to abort Sending 5,100-byte ICMP Echos to 131.108.3.2,timeout 2 seconds: !!!!! Table 9.2 shows some common X.25 show and debug commands WAN Technologies 15 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Table 9.2 The X.25 show and debug commands IOS Command Description show x25 map Displays the X.25 maps... Identifier FECN – Forward Explicit Congestion Notification BECN – Backward Explicit Congestion Notification DE – Discard Eligibility EA – Address Field Extension Figure 9.10 Frame Relay frame format (data) WAN Technologies 17 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Another feature of Frame Relay is that it implements some congestion control mechanisms Congestion Control... LMI frame format You are not expected to memorize the LMI frame format, but you need to be aware that there are two main frame formats in Frame Relay networks, as illustrated in Figures 9.10 and 9.11 WAN Technologies 19 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Field length in bytes 1 Flags 2 1 1 1 Variable Call Reference Nesting Type Data 1 Unnumbered Protocol LMI... 131.108.2.1 255.255.255.0 frame-relay interface-dlci 400 Listing 9.8 Configuring Frame Relay on remote routers (Sharon and Simon) Hostname Sharon interface serial 0 encapsulation frame-relay ○ Simon WAN Technologies 21 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ interface s0.1 point-to-point Description PVC to Simon ip address 131.108.1.2 255.255.255.0 frame-relay... a few Mbps to multi-gigabit speeds ATM cells are fixed in length of 53 bytes This byte size came about due to a compromise between voice and data experts Voice experts wanted 32 bytes (ideal size for voice sampling), and data experts wanted 64 bytes (ideal size for an network packet) Therefore, to satisfy both sides, the following equation was devised: 32+64=96/2+5 bytes of header gave 53 bytes As described... Interface (B-ICI)—A connection between public switches B-ICIC defines the protocols and procedures needed for establishing, maintaining, and terminating virtual connections between public networks WAN Technologies 23 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ Private UNI ○ Public NNI Private NNI ○ ILMI ○ Public ATM Network typically a Carriers Network Private ATM Network... and delay must be low to mai0ntain video and voice quality ➤ Unspecified Bit Rate (UBR)—Allows traffic to be sent as required but does not guarantee delivery Typically, UBR is used for data traffic WAN Technologies 25 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ➤ Variable Bit Rate (VBR)—Provides a limited amount of bandwidth VBR is useful for packet video and voice . program expands. WAN Technologies Wide area networks (WANs) provide connectivity across longer distances than local area networks (LANs), and WANs are typically. ○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○○ WAN Technologies ➤ ISDN ➤ PPP ➤ X.25 ➤ Frame Relay ➤ ATM Due the to high cost of WANs, Cisco introduced Dial on Demand Routing (DDR) to provide a WAN