Frame Relay

Frame Relay

Frame Relay Overview  Frame Relay Configuration  Frame Relay NBMA Configuration and Verification  Frame Relay Sub interface Configuration and Verification  Frame Relay Configuration Summary

Frame Relay Overview

Connection to a frame relay network is done with a local loop from the serial interface of a router to one of a service provider’s frame relay switches. Communication across a frame relay network uses virtual circuits, which are built by a service provider from a router’s serial interface, through a collection of frame relay switches, to another router’s serial interface. Virtual circuits that are programmed into a service provider’s network to stay active all the time are called permanent virtual circuits (PVCs). IOS also supports switched virtual circuits(SVCs), which become active only when they are used; however, SVCs are not yet widely available from frame relay service providers. We use only PVCs in this chapter. Many PVCs can be built on a single local loop. PVCs are addressed with Data Link Connection Identifiers (DLCIs) at layer 2. From our perspective, each PVC has two DLCIs—one at each end. From a router’s perspective, each PVC needs only one DLCI—the local one. When a router wants to transmit a packet to another router across a PVC, the router must know the local DLCI of the PVC on which the packet is to be transmitted. For this reason, some people say that DLCIs are locally significant. Figure 1 shows a basic frame relay network. There are three routers—Dallas, FortWorth, and Austin. Each router has a local loop to the frame relay network. There are two PVCs—one from Dallas to FortWorth and one from Dallas to Austin. Let us take a closer look at the PVC between Dallas and FortWorth. The Dallas end of the PVC has DLCI 100 and the FortWorth end has DLCI 101. These DLCIs, since they are on different local loops, do not have to be different, but they usually are, anyway. When Dallas wants to send a packet to FortWorth, Dallas must transmit the packet out the serial interface that contains the PVC, and the frame header must contain the local DLCI, 100. The switch knows that the path of the PVC with DLCI 100 on the Dallas side is supposed to go to the switch connected to FortWorth. The switch connected to FortWorth knows the path of the PVC extends across the local loop to FortWorth and the DLCI of the PVC on the local loop is 101. The switch puts the DLCI 101 into the frame header so the FortWorth router knows, when it receives the frame, that the frame came in on the PVC with local DLCI 101. Figure 1 - Basic Frame Relay Network Dallas has two PVCs coming in on the same local loop; therefore, these PVCs must have different local DLCIs. The frame relay topology shown in Figure 1 is called a partial-meshednetwork because not all of the routers have PVCs to all of the other routers. We could also call this particular topology a hub-and-spoke network because there is one router (the hub) that has a connection to each of the other routers (the spokes), and traffic from a spoke router must go through the hub to reach another spoke router. A fully meshed network has PVCs running between all of the router pairs; to make the Figure 1 frame relay topology into a fully meshed network, we would have to add a PVC between FortWorth and Austin. Routers and switches maintain contact with each other using Local Management Interface(LMI). About every 10 seconds, routers and switches send an LMI keepalive across the local loop. If a router is receiving LMI keepalives from a switch, the router makes the line protocol of its interface up so the interface state will be up/up. If a Cisco router interface is connected to a frame relay network and the state of the interface is up/up, the router has a communication path to a frame relay switch. This has nothing to do with being able to reach another router on the other end of a PVC. By default, a Cisco router uses LMI to request a status report from the switch every six keepalives (about once a minute). The status report contains a list of each of the local loop’s PVCs, their DLCIs, and their status. There are two types of LMI that are widely used between routers and switches: Annex D and Gang of Four. Annex D is from the American National Standards Institute (ANSI). The Gang of Four LMI was jointly developed by Cisco, DIGITAL, Intel, and Stratacom. The router and the local switch must agree on the type of LMI they will use between them.