Frame Relay Overview

   

Frame Relay is a very popular technology for connectivity across WAN links. Before configuring the interfaces, you probably should gain a basic familiarity with Frame Relay. We will not cover Frame Relay technology in depth; we will just review the essentials so that you understand how Frame Relay is operating in the lab for configuration and troubleshooting tasks . For a complete review of Frame Relay, see Chapter 13 of Interconnecting Cisco Network Devices from Cisco Press.

Frame Relay is a physical and data link layer encapsulation technology, as depicted in Figure 7-2.

Figure 7-2. Frame Relay and OSI Reference Model Correlation

graphics/07fig02.gif

Frame Relay is an ITU-T (CCITT) and American National Standards Institute (ANSI) defined standard that outlines sending data over a public network.

Frame Relay Components

You should be familiar with several Frame Relay components before going through the Frame Relay interface configuration process:

  • Data-link connection identifier (DLCI) This is a number that identifies the logical circuit between the router and the Frame Relay switch. The Frame Relay switch maps the DLCIs between each pair of routers to create a PVC. DLCIs have local significance in that the identifier references the point between the local router and the Frame Relay switch to which it is connected. Thus, the same (or different) DLCI numbers can be used on both ends and it still would work properly.

  • Permanent virtual circuit (PVC) This is a virtual circuit that is permanently established. A PVC is an end-to-end path (that is, router-to-router path through a Frame Relay cloud). One or more DLCIs form a PVC in a Frame Relay network. PVCs save bandwidth associated with the establishment and teardown of circuits.

  • Local Management Interface (LMI) This is a signaling standard between the router device and the Frame Relay switch that is responsible for managing the connection and maintaining status between the devices.

NOTE

Not all Frame Relay terminology is reviewed here. Frame Relay is a useful technology, and there are several different applications of Frame Relay. Again, we will review only what is needed to complete the lab.


Frame Relay Address Mappings

An important part of Frame Relay is the mapping of IP addresses to DLCIs. Two methods exist for Cisco routers to form these mappings:

  • Frame Relay Inverse ARP (dynamic) This is a method of dynamically associating a network layer address with a DLCI. It allows a router to discover the network address of a device associated with a DLCI. Inverse ARP normally is used in fully meshed networks (networks in which all routers have PVCs to each other). The environment in this lab is a hub-spoke topology, so you will encounter some limitations with Frame Relay Inverse ARP.

  • Static (manual) mappings In this method, you manually map a network layer address, such as IP or IPX, to a DLCI. This is done under the interface configuration mode for the serial interface configured for Frame Relay. It is important to note that when you manually map an IP address to a DLCI, Frame Relay Inverse ARP is disabled for that interface. You will have an opportunity to manually map IP addresses to DLCIs in this lab.


   
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CCNA Practical Studies
CCNA Practical Studies (Cisco Certification & Training)
ISBN: 1587200465
EAN: 2147483647
Year: 2005
Pages: 127

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