Cisco 6500
Cisco 6500 Series Ethernet Switches        All of the Catalyst 6500 series Ethernet modules implement some form of receive and transmit buffering. These buffers are used to store frames as forwarding decisions are made within the switch, or as packets are enqueued for transmission on a port at a rate greater than the physical medium can support. In the Catalyst 6500 architecture, access into the switch fabric itself is almost never the bottleneck. Rather, on the transmit side, one or several ports are the likely destination for a majority of the packets entering the switch. As such, the receive-side port buffers on the Ethernet modules are relatively small compared to the transmit-side port buffers.
Without Quality of Service (QoS) features enabled, all packets have equal access to the port buffers, regardless of the type of traffic. For example, Voice over IP (VoIP) packets get placed in the same buffer as HTTP (web) or FTP (file download) data packets. Furthermore, in the event of congestion (that is, a port buffer overflows), all traffic is equally subject to discard. To summarize, packets are serviced in the order in which they were received, and if the buffer is full, all subsequent packets are dropped. This is known as First In, First Out (FIFO) queuing with tail-drop.
        When you enable QoS on the switch, the port buffers are carved into one or more individual queues. Each queue has one or more drop thresholds associated with it. The combination of multiple queues within a buffer, and the drop thresholds associated with each queue, allow the switch to make intelligent decisions when faced with congestion. Traffic sensitive to jitter and delay variance, such as VoIP packets, can be moved to the head of the queue for transmission, while other less important or less sensitive traffic can be buffered or dropped. When QoS is enabled, the multiple queues and drop thresholds on the Ethernet module switch ports are enabled. There are several different configurations of queue types and thresholds, depending on the model of the Ethernet module.

Buffers, Queues & Thresholds on Catalyst 6500 Ethernet Modules

Receive and Transmit Port Queue and Drop Threshold Types on Catalyst 6500 Series Ethernet Modules

Port Queue and Drop Threshold Structure with QoS

Description

Receive Queues

1q2t

One standard queue with two tail-drop thresholds

1q4t

One standard queue with four tail-drop thresholds

1q8t

One standard queue with eight tail-drop thresholds

2q8t

Two standard queues with eight tail-drop thresholds per queue

8q4t

Eight standard queues with four WRED drop thresholds per queue

8q8t

Eight standard queues with eight WRED drop thresholds per queue

1p1q4t

One strict-priority queue, one standard queue with four tail-drop thresholds

1p1q0t

One strict-priority queue, one standard queue with one non-configurable (100%) tail-drop threshold

1p1q8t

One strict-priority queue, one standard queue with eight configurable WRED drop thresholds and one non-configurable (100%) tail-drop threshold

Transmit Queues

2q2t

Two standard queues with two tail-drop thresholds per queue

1p2q2t

One strict-priority queue, two standard queues with two WRED drop thresholds per queue

1p3q1t

One strict-priority queue, three standard queues with one WRED drop threshold and one non-configurable tail-drop threshold per queue

1p2q1t

One strict-priority queue, two standard queues with one WRED drop threshold and one non-configurable (100%) tail-drop threshold per queue

1p3q8t

One strict-priority queue, three standard queues with eight WRED drop thresholds per queue

1p7q8t

One strict-priority queue, seven standard queues with eight WRED drop thresholds per queue

1p7q4t

One strict-priority queue, seven standard queues with four WRED drop thresholds per queue