Input Scheduling

Input scheduling is the next step involved in handling the frame after the frame has arrived at the ingress port, assuming the port has been configured for trust-cos (refer to Figure 8-1). Input scheduling basically assigns incoming frames to queues. If trust-cos is not configured, the incoming frames will bypass the Receive threshold (also known as the drop threshold) queue and are forwarded directly to the switching engine. Each queue has its own drop threshold level, which means that frames are dropped after the threshold value is exceeded.

The number of queues and their associated drop threshold values are dependent on the hardware used. Example 8-4 shows features available for port 10/3 off the WS-X6248-xx module. Note the QoS scheduling field shaded in the example. There are two defined queues: rx-(1q4t),tx-(2q2t). Input scheduling deals with rx-(1q4t). The tx-(2q2t) will be discussed later in the chapter. The 1q4t is defined as 1 queue with 4 drop thresholds. Newer line cards have 1p1q4t, translating to 1 priority queue, 1 normal queue, with 4 drop threshold queues. Each of these thresholds is set to drop incoming packets. The packets are dropped based on their CoS setting and the amount of buffer used.