Icnd210 s08l03

Editor's Notes

• #3: <number> Purpose: This figure provides a big-picture definition of Frame Relay. Emphasize: Frame Relay is used between the CPE device and the Frame Relay switch. It does NOT affect how packets get routed within the Frame Relay cloud. Frame Relay is a purely Layer 2 protocol. The network providing the Frame Relay service can be either a carrier-provided public network or a network of privately owned equipment serving a single enterprise. Make a clear distinction between DCE, DTE, and CPE. Emphasize that Frame Relay over SVCs is not discussed in this chapter because it is not widely supported by service providers at this time. The service provider must also support SVCs in order for Frame Relay over SVCs to operate. Note: In Cisco IOS Release 11.2, two traffic shaping features were introduced: Generic (adaptive) traffic shaping Frame Relay traffic shaping Both of these features can be used to adjust the rate at which traffic is sent by the router. In addition, these features allow the router to throttle the traffic rate based on BECNs received from the Frame Relay switch. Neither of these features are discussed in this course. Frame Relay traffic shaping is discussed in the Building Cisco Remote Access Networks (BCRAN) course. Information on both can be found in Cisco documentation.
• #4: <number> Purpose: This figure provides an overview of terminology so that the student is prepared to understand the Frame Relay operation discussion. The terminology used with Frame Relay varies by service provider. These are the commonly used terms. Point out the local access loop and note that the local access rate is different than the rate used within the Frame Relay cloud. The DLCI is of local significance, therefore, point out that the same DLCI can be used in multiple places in the network. The autosensing LMI is a Release 11.2 or later feature. Frame Relay connections are made using PVCs. The circuits are identified by the DLCI assigned by the service provider. Reference: For more information on Frame Relay, including a Frame Relay glossary, refer to the Frame Relay Forum World Wide Web page: http://www.frforum.com/4000/4003.html This course does not discuss Frame Relay traffic flow issues. Terms like BECN, FECN, and discard eligible are not discussed in this course. These terms are some of the terms that can be found in the Frame Relay Forum’s glossary. The BCRAN discusses Frame Relay traffic flow issues.
• #5: <number> 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 NBMA Frame Relay network. 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 a problem when multiple PVCs are supported by a single interface. In this situation, broadcast routing updates received by the central router cannot be broadcast to the other remote sites. Broadcast routing updates are issued by distance vector protocols. Link-state and hybrid protocols use multicast and unicast addresses.
• #6: <number> Purpose: This figure defines subinterfaces and how they can resolve NBMA issues. Emphasize: You can have connectivity problems in a Frame Relay network if the following 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 update, this split horizon will keep this update from being sent back out the subinterface to router D. To resolve this problem you can do the following: 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.
• #7: <number> Purpose: This figure illustrates mapping the data-link connection identifier (DLCI) to the network-layer address such as IP. Emphasize: The DLCI is of local significance, therefore, point out that the same DLCI can be used in multiple places in the network. Frame Relay connections are made using PVCs. The circuits are identified by the DLCI assigned by the service provider. Explain what Inverse ARP is used for. Static mapping can be configured instead of Inverse ARP.
• #8: <number> Purpose: This figure describes the Local Management Interface (LMI) and shows the key standards. Emphasize: Explain LMI. Note: Other key American National Standards Institute (ANSI) standards are T1.606, which defines the Frame Relay architecture, and T1.618, which describes data transfer. Other key International Telecommunication Union Telecommunication Standardization Sector (ITU-T) specifications include I.122, which defines ITU-T Frame Relay architecture, and Q.922, which standardizes data transfer. Use of these LMI standards is especially widespread in Europe. The original “gang of four” no longer exists; StrataCom® merged with Cisco, and Digital Equipment Corporation was acquired by Compaq Computers.
• #9: <number> 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.
• #10: <number> Layer 3 of 3 Purpose: This figure describes the Inverse ARP and LMI process. 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.
• #11: <number> 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 ietf encapsulation command 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 be required in case of intermittent failures. Changing the default values of the LMI should only be attempted after consulting with your service provider. These configuration steps are the same, regardless of the network-layer protocols operating across the network.
• #12: <number> 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 the correct DLCI. 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.
• #13: <number> 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.
• #14: <number> 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 which dynamic address resolution is enabled. It is not used for multipoint subinterfaces configured with the frame-relay map command for static address mapping. 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 a network outage. Create another subinterface in the software and migrate the configuration parameters to the new subinterface using the proper point-to-point or multipoint setting, as required.
• #15: <number> 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.
• #16: <number> 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.
• #17: <number> 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.
• #19: <number> 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.
• #20: <number> 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.
• #21: <number> Lab 16 Frame Relay Objectives: Enable the Frame Relay on a serial link. Purpose: Teach students how to enable Frame Relay. Laboratory Instructions: Refer to the lab setup guide.