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Frames are buffered in the I/O modules, before and after traversing the switch. Each queue can hold 256K bytes. (Architecturally they can support up to 1 MB each).
Each buffer is divided into two queues, one for high-priority traffic and one for normal-priority traffic. The default is for the high-priority queue to use 20% (51K) of the buffer. The normal-priority queue uses the remaining 80% (205K). These values can be modified using either the Web Agent or SNMP.
Note: After changing these values, you must reboot the switch for the changes to take effect.
Less buffer memory is assigned to the high-priority queue because the high-priority queue gets serviced more frequently than the normal-priority queue. Since a frame spends less time on the high-priority queue, less buffer space is required for the queue.
The Service Ratio can be chosen to match traffic patterns and performance requirements using a weighted round robin scheduling algorithm. The available service ratios of the algorithm are defined in "Managing Buffers and Queues". The factory default service ratio for fabric ports is 999/1. The factory default service ratio for physical ports is 1023 to 1. If traffic from both the high- and normal-priority queues must be serviced, 999 packets of high-priority traffic will be processed for each normal-priority packet.
When the high-priority queue fills up, incoming frames are dropped. The philosophy is if a high-priority frame is going to be late, it is not worth sending it at all. The normal-priority queue uses either IEEE 802.3X PAUSE (variable timed XOFF) flow control or Half Duplex collisions to shut off incoming frames before the queue overflows.
The switch implements two flow control disciplines along the entire path that frames travel. The default case is that when output buffers fill up, frames destined for a particular buffer will be dropped. This should only occur in a case where the output port is very congested. However, there is an optional mode in which normal-priority frames are never dropped inside the switch. In this mode, input buffers may fill up. If they do, the affected input ports may use flow control to temporarily halt traffic from neighboring switches.
Note: 802.1p packets that are received with a tag priority of 0 on a 50-series layer 2 (non-routing) module, and that must be routed via the FORE path on an 80-series supervisor, are queued and transmitted with a priority of 4. This priority change is due to the conversion from the high-low priority system that 50-series modules use to the 8-level priority system that 80-series modules use.
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