CISCO IOS in a Nutshell

Chapter 6. Frame Relay and ATM

Frame Relay

In the past decade, Frame Relay has become a popular wide-area network (WAN) switching method. Building a WAN by creating virtual circuits inside a provider’s Frame Relay network has become a much more attractive option than ordering leased lines between the locations you want to connect. Instead of paying for all those leased lines, you just pay for access to the provider’s network. Of course, there is still a leased line to the provider, but it is typically fairly short—much shorter than a leased line between your end locations. It’s easy to order the bandwidth you need for each link; furthermore, Frame Relay allows you to reserve a guaranteed minimum bandwidth (called the “committed information rate” or CIR) but lets you use as much bandwidth as is available on the network. In fact, you don’t need the same speed circuits at all your sites. For example, your corporate headquarters might use a DS-3 (45-Mbps line), while your regional offices all have T1 lines.

At the physical level, a Frame Relay connection looks just like a serial interface—because it is. A standard leased line (typically a T1 line) connects your site to the Frame Relay provider. Although it’s more complex than a simple serial interface, the complexity comes mostly from mapping the IP addresses of the nodes on your network into Data Link Connection Identifiers (DLCIs), which are the Frame Relay equivalent of addresses.

Important Frame Relay Terminology

Before we look at ...