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260 CHAPTER 17: CONFIGURING FRAME RELAY
notification (BECN) flag bit is 1, indicating that the congestion has occurred to the
network, and the transmit rate of the PVC will be lowered to CIR.
Frame Relay Queueing Management
To ensure that the packets on the PVCs can be transmitted at an average rate in
the process of Frame Relay traffic shaping, a queueing mechanism should be
adopted to manage the packets. Generally, except that the Frame Relay interface
owns one interface queue, the Frame Relay PVC does not have its own transmit
queue. However, after the Frame Relay traffic shaping is enabled on the Frame
Relay interface, all the PVCs belonging to this interface will own their independent
queues, and all the packets t transmitted from the PVCs will enter the Frame Relay
PVC queues first.
Besides the Frame Relay PVC queues, the Frame Relay interface also owns an
interface queue. In the case that the Frame Relay traffic shaping is not enabled,
there will only be the Frame Relay interface queue. After it is enabled, both Frame
Relay PVC queues and the Frame Relay interface queue will exist. Their relations
are illustrated in
Frame Relay queueing.
Figure 84 Frame Relay queueing
The Frame Relay PVC queueing types include FIFO (First-In First-Out Queueing), PQ
(Priority Queueing), CQ (Custom Queueing), and WFQ (Weighted Fair Queueing).
The FIFO, PQ, CQ, WFQ, and PIPQ (PVC Interface Priority Queueing) queues can be
used on a Frame Relay interface. Among them, FIFO, PQ, CQ, and WFQ queues
are general queues. For the detailed introduction, refer to the part of
QoS. PIPQ
can only be applied on the Frame Relay interface. It is similar to PQ, but aiming at
the PVCs on an interface. When the Frame Relay traffic shaping is enabled on an
interface, the queueing type on the interface can only be either FIFO or PIPQ.
PIPQ is applied on a Frame Relay interface. There are four types of PIPQ: top,
middle, normal and bottom. Their queueing priorities are listed in descending
order. The packets on the same PVC can only enter one type of PIPQ queue, and
the packets on different PVCs enter different PIPQ queues on the interface,
depending on the priorities of the PVCs. The PIPQ transmitting policy is as follows:
Based on the queueing priority, transmit the packets in the queue with low priority
after those in the queue with high priority are transmitted.
Frame Relay Traffic Policing
Frame Relay policing monitors the traffic that flows into the network from each
PVC, and restricts it in a certain range. If the traffic size on a PVC exceeds the
range set by the user, the router will adopt the measures like discarding the
packets, so as to protect the network resources and the profit of the operator.
PVC Queue
Interface queue