Lucent Technologies PortMaster - Manual (Page 209)

Using Frame Relay 13-3
Overview of Frame Relay
Discarding Frames
The PortMaster pushes as much data out of the serial port as it can at port speed for any
PVC that has traffic, regardless of CIR. The Frame Relay switch passes as much of the
data as possible on to the next link. However, once a particular PVC has transmitted its
CIR-worth of bits each second, the switch marks any additional frames as “discard
eligible.” If the switch receives more frames than it can pass along, the frames are
automatically discarded in the following order:
• Frames that would be marked discard eligible even if they are forwarded
• Frames received that were marked as discard eligible
If the switch must discard other frames, the behavior is undefined. In this case, the
Frame Relay network is improperly configured because the CIR total exceeds the line
speed or port speed.
Ordering Frame Relay Service
In general, when ordering Frame Relay service for a private network, order
large-bandwidth physical circuits (T1) with port speed appropriate to your application,
and a CIR that is high enough to provide minimally acceptable performance for your
application. In most cases, ordering according to these criteria provides service that is
close to your port speed. The CIR is a guaranteed minimum throughput, not a
maximum limit. Port speed is the maximum limit.
LMI Types
The following Frame Relay terms relate to network management. The Frame Relay
specification supports automatic network status updates, which are exchanged between
adjacent devices in the Frame Relay network. These status updates are known as the
Local Management Interface (LMI). Two forms of LMI are available in the PortMaster:
Cisco/Stratacom LMI, which is commonly referred to as LMI, and ANSI T1.617 Annex D
LMI, which is commonly referred to as Annex-D.
Generally, your telecommunications provider offers three LMI options for your physical
circuit: LMI, Annex-D, or none. Because LMI exists only between your router and the
switch to which your physical circuit connects, it does not need to match what the
remote ends of your PVCs are using. However, your circuit LMI must match the
configuration on your PortMaster. Generally, Annex-D is recommended, because it is a
more feature-rich and robust version of LMI.