Frame Relay Components - Page 2

By Cisco Press | Posted Jan 10, 2002
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Frame Relay Virtual Circuits

Frame Relay is a connection-oriented service, operating at the data link layer (Layer 2) of the OSI model. A DLCI is used to identify this dedicated communication path between two end nodes: origination and termination. This path, or VC, is a bidirectional logical connection across the wide-area network between two end node DTE devices.

Figure 15-5 illustrates a fully meshed (all sites with connectivity to each other) Frame Relay WAN, with DLCI assignments for each location.


NOTE:   Sometimes the originating node of a VC will be annotated as Site A and the terminating node of a VC will be annotated as Site B or Site Z.

Figure 15-5: Frame Relay WAN with Virtual Circuit and DLCI
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(Click image for larger view in a new window)

DLCIs
DLCIs are used to identify the PVC that is provisioned to transport data traffic. DLCIs are of local significance, unless an agreement has been made with the network service provider to deploy global DLCIs. Local significance means that DLCIs are of use only to the local Frame Relay network device. Frame Relay DLCIs are analogous to an organizations telephone network that is utilizing speed-dial functions. The most common Frame Relay WAN deployment involves the use of local DLCIs because a network size limitation exists for the use of global DLCIs.


NOTE:   Global DLCI addresses are assigned so that each DLCI has universal significance, meaning that the DLCI number is pointed to the same destination (termination point) regardless of the origination point.

The concept behind global DLCI addressing is to simplify Frame Relay network addressing administration; however, global addressing has an inherent limitation in that no more than 992 DLCIs (1024 DLCIs less the 32 reserved DLCIs) can be used. In a Frame Relay network of more than 992 sites, global addressing will not work.

The use of global DLCIs requires that they each be preassigned. (Typically, the assignments are negotiated between the customer and the network service provider.) In addition, each DLCI can be used only once throughout the network. (If two sites had the same DLCI, the network would not know which termination site was the intended destination.) The Frame Relay switch within the network service providers network will have tables that route the traffic between each origination and termination pair.

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