3Com MSR 50 Series Configuration Manual page 1715

the queuing priority, transmit the packets in the queue with low priority after
those in the queue with high priority are transmitted.
Frame Relay virtual circuit queuing may be FIFO, PQ, CQ, WFQ, CBQ and RTPQ.
Only RTPQ can coexist with others.
When Frame Relay traffic shaping is enabled, only FIFO, RTPQ or PVC PQ is
available.
Frame Relay congestion management
Frame Relay congestion management can process the Frame Relay packets
whenever there is network congestion. It will discard the packets that are marked
with the DE flag bit 1. At the same time, it will notify other devices on the network
about the congestion.
Frame Relay congestion management is applied at the egress interface on a Frame
Relay switched device. If there is no congestion, the device will normally forward
the Frame Relay packets without doing any special processing on them. Once
there is congestion, the packets that are marked with the DE flag bit 1 will be
discarded. As for the forward packets to be forwarded, the device will set the
FECN flag bit in the Frame Relay packet headers to 1. As for the backward packets
on the same PVC, the device will set the BECN flag bit in the Frame Relay packet
headers to 1. If there is no backward packets to be forwarded after a certain
period, the device will automatically transmit the Q922A Test Response packets
with the BECN flag bit 1 to the calling DTE.
Figure 494 Frame Relay congestion management
Frame Relay DE rule list
In a Frame Relay network, the packets that are marked with DE flag bit 1 will
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be first discarded once there is congestion. The DE rule lists are applied on the
Frame Relay PVCs on a router, and each of them contains multiple DE rules. If a
packet transmitted on a PVC complies with the rules in the DE rule list, its DE
flag bit will be set to 1, and the packets like it will be discarded first if the
congestion occurs on the network.
FR WRED
Currently, only AF and BE queues in WFQ and CBWFQ of FR QoS support WRED.
The EF queue in CBWFQ does not support WRED for the time being.
Frame Relay QoS Overview 1715