In addition to the common LMI features,
several optional LMI extensions are extremely useful in an
internetworking environment. The first important optional LMI
extension is global addressing. With this extension, the values
inserted in the DLCI field of a frame are globally significant
addresses of individual end-user devices (for example, routers).
As noted earlier, the basic (nonextended)
Frame Relay specification supports only values of the DLCI field that
identify PVCs with local significance. In this case, there are no
addresses that identify network interfaces, or nodes attached to these
interfaces. Because these addresses do not exist, they cannot be
discovered by traditional address resolution and discovery techniques.
This means that with normal Frame Relay addressing, static maps must
be created to tell routers which DLCIs to use to find a remote device
and its associated internetwork address.
In the Figure, note that each interface has its own
identifier. Suppose that Pittsburgh must send a frame to San Jose. The
identifier for San Jose is 22, so Pittsburgh places the value 22 in
the DLCI field and sends the frame into the Frame Relay network. Each router interface has
a distinct value as its node identifier, so individual devices can be
distinguished. This permits routing in complex environments. Global
addressing provides significant benefits in a large, complex network.
The Frame Relay network now appears to the routers on its periphery
like any LAN.
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