Frame Relay defines the rules by which devices deliver Frame Relay frames across a Frame Relay network.  Because a router uses a single access link that has many VCs connecting it to many routers, there must be something to identify each of the remote routers – in other words, an address.  The DLCI is the Frame Relay address.

DLCIs work slightly different from the other data-link addresses covered on the CCNA exams.  This difference is mainly because of the use of the DLCI and the fact that the header has a single DLCI field, not both Source and Destination DLCI fields.

Frame Relay Local Addressing

Frame Relay DLCIs are locally significant; this means that the addresses need to be unique only on the local access link.  DLCIs must be unique on each access link, but the same DLCI numbers can be used on every access link in your network.

Local addressing, which is the common term for the fact that DLCIs are locally significant, is a fact.  It is how Frame Relay works.  Simply put, a single access link cannot use the same DLCI to represent multiple VCs on the same access link.  Otherwise, the Frame Relay switch would not know how to forward frames correctly.

Frame Relay Global Addressing

Global addressing is simply a way of choosing DLCI numbers when planning a Frame Relay network so that working with DLCIs is much easier.  Because local addressing is a fact, global addressing does not change these rules.  Global addressing just makes DLCI assignment more obvious.

Here’s how global addressing works: The service provider hands out a planning spreadsheet and a diagram.  Global addressing is planned with the DLCIs placed in Frame Relay frames.  The nice thing is that global addressing is much more logical to most people, because it works like a LAN, with a single MAC address for each device.  On a LAN, if the MAC addresses are MAC-A, MAC-B, and MAC-C for the three routers, router A uses destination address MAC-B when sending frames to Router B and uses MAC-C as the destination to reach Router C.  The beauty of global addressing is that you think of each router as having an address, like LAN addressing.  If you want to send a frame to someone, you put his or her DLCI in the header, and the network delivers the frame to the correct DTE.

The final key to global addressing is that the Frame Relay switches actually change the DLCI value before delivering the frame.

* The sender treats the DLCI field as a destination address, using the destination’s global DLCI in the header.

* The receiver thinks of the DLCI field as the source address, because it contains the global DLCI of the frame’s sender.