Global DLCI addressing
 
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Last time, we explained how addressing generally works in a frame relay network.
Most approaches in place today use locally significant Data Link Connection Identifiers, or addresses, at each end of a connection and leave it up to the service provider's frame relay switch to map that DLCI to the one at the receiving end. The main benefit of this approach is that the same address can be used in any number of sites. This means that a greater number of addresses are available as networks grow than if sites are assigned a globally unique DLCI address.
In the original Frame Relay Forum addressing specifications, however, a global addressing scheme also was sanctioned to mimic addressing used in Layer 3 routers. In a global addressing scenario, a fixed DLCI is assigned to a specific location. When the router constructs the header of an outbound frame, it inserts the DLCI value of the destination (rather than a DLCI value from the local pool of unused numbers). Such a scheme was thought to help in the area of troubleshooting, because a network technician can look at a DLCI value and know immediately if a frame is destined for a particular location.
The downside is that it has a size ceiling, because of the limited size of the DLCI field. There are only 976 addresses available out of the 1,024 combinations in the 10-bit DLCI, because 48 bits have been reserved for other functions, such as Local Management Interface signaling and other management functions. So, if a network grows larger than 976 sites, the customer will run out of unique DLCIs. Also, the global approach gets confusing if there is more than one virtual circuit running between two locations. Depending on the service provider's switch implementation, the switch may or may not be able to distinguish between them.
If you are involved in setting up DLCIs for your frame rely sites, it can be helpful to ask your carrier which approach to addressing is supported in the carrier's switches - local or global - so that you use the scheme supported by your provider.
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Steven Taylor, consultant and broadband packet evangelist, and Joanie Wexler, an independent networking technology editor and writer, team up to bring you this analysis and commentary. Taylor specializes in education and market analysis, and Wexler adds incisive reporting and research. For more detailed information on most of the topics discussed in this newsletter, connect to www.webtorials.com, the first Web site dedicated exclusively to market studies and technology tutorials in the Broadband Packet areas of Frame Relay, ATM, and IP.
Feedback and additional topic ideas are welcome. Please contact taylor@webtorials.com or joanie@jwexler.com.
Frame Relay archive
Past newsletters.
Network World, 12/14/98.
Review: SLA enforcement tools to the rescue
Network World, 04/03/00.
Newsletter: How secure is frame relay?
Network World Fusion Focus, 06/30/99.
