The 7705 SAR uses an Ethernet-based fabric. Each packet that is sent to the fabric is equipped with a fabric header that contains its specific CoS requirement. Since all of the packets switched across the fabric are already classified, queued, scheduled and marked according to the desired QoS parameters, each of these packets has been passed through the Traffic Management (TM) block on an adapter card, or the Control and Switching Module (CSM) in the case of control packets. Therefore, each packet arrives at the fabric having been already scheduled for optimal flow. The function of the fabric is to switch each packet through to the appropriate destination adapter card, or CSM in the case of control packets, in an efficient manner.
Since the traffic is shaped at a certain rate by the ingress adapter card (that is, bursts are smoothed by the traffic management function), minimal buffering should be needed on the switch fabric. However, the buffer space allocation and usage is in accordance with the priorities at the ingress adapter card. As is the case with schedulers at the adapter cards, there are two priorities supported on the switch fabric. The switch fabric serves the traffic in the following priority order:
Expedited
Best Effort
The switch fabric does not support profile scheduling.
Since the fabric has a limited buffer space, it is possible for tail drop to occur. Tail drop discards any packet that exceeds the maximum buffer space allocation. The shaping that is performed on the adapter cards helps to prevent or minimize congestion.