Lets say that you need a 15Mbit pipe but for one reason or another, just can't get a fractional DS-3. You could get 10 T1 circuits or 10 1.5Mbit frame relay lines and let the routing protocol handle the load balancing. Of course, this means that you'd have a bunch of sub interfaces and IP addresses that need to be dealt with, in addition to the routing traffic and processing delays.

Multilink frame relay solves that problem. A bundle is configured on each side of the WAN with the layer three information, and the interfaces deal with the frame relay traffic themselves. The type of interface that needs to be created is, appropriately enough, an "mfr" or multilink frame relay interface. You can also name the interface by assigning a "bid" or bundle ID.

Router(config)# interface mfr1
Router(config-if)# frame-relay multilink bid Chicago

Just like normal interfaces, the mfr interface needs to be brought up with the "no shutdown" command.

Frame relay doesn't strictly operate on the physical interface anymore; instead, it operates on the logical mfr interface. Each physical interface that needs to participate in the bundle needs to be told which bundle it is now a part of. The way this is done is a slight modification in the traditional method of applying Frame Relay encapsulation to the interface:

Router(config)# interface serial 0/1
Router(config-if)# encapsulation frame-relay mfr1

In addition to telling the interface that it is now a member of the bundle, you can give it instructions on how to act. Instructions like how many bytes to transfer before rolling the traffic over to the next circuit, a final form of load balancing so that one circuit doesn't get all the traffic when there isn't much. You can also set timers for hellos, acks and retries of the hello messages.

Functional use of multiple frame relay circuits comes down to either a bandwidth issue or a quality of service (QoS) issue. Bandwidth is easy: If your bandwidth increases, you can transfer more information. If you had a bottleneck at this point, it has either been reduced, or moved to another location. There's always a bottleneck, only "where" changes.

Multilink Frame relay can help the QoS of the network in some cases, especially where there is delay sensitive traffic. If there is a single 15Mbit circuit then traffic needs to wait its turn to access the line. Higher priority traffic still waits if the lower priority traffic was already accessing the line. If there are several 1.5Mbit circuits that can be used though, there is little waiting. High priority traffic just moves on to the next available open circuit and gets going across the WAN. There is less of a need for fragmentation because large frames are less likely to cause delays to time sensitive frames.