FRTS shape to 95% of CIR

Frame Relay Dual-FIFO

On the low-end router non-distributed platforms (Cisco 7200 and lower), Frame Relay employs a dual-FIFO queuing technique that automatically is invoked at the interface level when FRF.12 is configured. FRF.12 depends on Frame Relay traffic shaping (FRTS) or class-based FRTS being enabled.

In a Frame Relay environment, the Tx-ring does not directly provide back pressure to the Layer 3 queuing algorithm. Instead, when the Tx-ring is full, it provides back pressure to the shaper (FRTS or CB-FRTS), which, in turn, signals the Layer 3 queuing system (LLQ) to engage. Because the FRTS mechanism does not take into account Frame Relay headers and cyclic redundancy checks (CRCs) in its calculations, it generally is recommended that you shape to 95 percent of CIR on Frame Relay circuits up to T1/E1 speeds. This, in turn, engages the LLQ algorithm slightly earlier and improves performance for real-time traffic.

Traffic from each PQ for each DLCI is funneled into the high-priority, dual-FIFO interface queue; all traffic from the CBWFQ queues from the DLCIs is assigned to the lower-priority, dual-FIFO interface queue. Thus, the dual-FIFO Layer 2 queues ensure that the "priority" class traffic from one DLCI is not delayed by CBWFQ traffic from another DLCI.

資料來源:Cisco Press End to End QoS Network Design Quality of Service in LANs WANs and VPNs
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