R2511-HP MSR Router Series ACL and QoS Configuration Guide(V5)

107
• FIFO
• PQ
• CQ
• WFQ
• CBQ
• RTPQ
• PVC PQ
Of these queuing mechanisms, FIFO, PQ, CQ, WFQ, CBQ, and RTPQ are universal queuing
mechanisms. For more information, see "Configuring congestion management."
PVC PQ can only be applied on FR interfaces. PVC priority queues include the following types: top,
middle, normal, and bottom, in the descending priority order. The packets from a certain PVC must be
assigned to one PVC priority queue, and the packets from PVCs are assigned to PVC priority queues on
the interface as configured. PVC PQ dequeues and sends packets in the high-priority queue and then
packets in the low-priority queue.
FR PVC queuing mechanisms include FIFO, PQ, CQ, WFQ, CBQ, and RTPQ. Only RTPQ can coexist
with another queuing mechanism.
With FRTS enabled on an interface, only FIFO, RTPQ, or PVC PQ is available on the interface.
FR congestion management
FR congestion management can process FR packets when congestion occurs in the network. It drops the
packets with the DE flag bits set to 1 and notifies other devices on the network about the congestion.
FR congestion management is applied on the outgoing interface of an FR switching device. If no
congestion occurs, the FR switching device forwards the FR packets without any processing. If congestion
occurs, packets with the FE flag bits set to 1 are dropped. As for forward packets to be forwarded, the
FECN flag bits in the FR packet headers are set to 1. As for backward packets on the same PVC, the
BECN flag bits in the FR packet headers are set to 1. If no backward packets are transmitted within a
period, the device automatically transmits the packets with the BECN flag bits set to 1 to the calling DTE.
Figure 47 FR congestion management implementation
FR DE rule list
In an FR network, packets with the DE flag bits set to 1 are dropped first when congestion occurs in the
network. DE rule lists are applied on the FR PVCs of a device, with each DE rule list containing multiple
DE rules. If a packet transmitted over the PVC matches the rules in the DE rule list, its DE flag bit is set to
1. The packet is dropped first when congestion occurs in the network.
帧中继网络
DTE
DCE
NNI
Router A
Router B
Data flow direction
BECN
FECN
Frame-Relay
network
Frame relay
network