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packets; however, the first routed packet loses the original CoS marking. All subsequent packets are switched by the PFC and retain their original Layer 2 and Layer 3 markings.

Although the PFC and PFC2 are both Layer 3 switching engines, they differ in a few ways, as listed in Table 10-5.

Table 10-5 PFC and PFC2 Differences

After a flow has been established, the PFC has the capability to use a QoS access list to police traffic to reduce the flow of traffic to a predefined limit. Traffic in excess of that limit can be dropped or have the DSCP value in the frame marked down to a lower value.

A QoS access-control list (ACL), consisting of a list of acess-control entries (ACEs), defines a set of QoS rules that the PFC uses to process incoming frames. ACEs are similar to a router ACL. The ACE defines classification, marking, and policing criteria for an incoming frame. If an incoming frame matches the criteria set in the ACE, the QoS engine processes the frame.

Ethernet Interfaces

The QoS role of Ethernet interfaces in a Catalyst 6500 includes the scheduling of packets and congestion management, as well as providing inline power to support the addition of real-time applications such as IP telephony. It is important to understand the capabilities of the Ethernet modules in your Catalyst to properly provision your network for the addition of real-time applications. Table 10-6 illustrates the QoS advantage that the current generation

of Ethernet modules, typically the WS-X65xx series of cards, offers over the past generation of

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Example 10-5 The show port capabilities Command (Continued)

The WS-X65xx series of Ethernet cards introduce a priority queue and extend the buffer sizes to better accommodate traffic during times of congestion. In a side-by-side comparison, it is plain to see that the WS-65xx series of Ethernet line cards are the better choice for QoS.

The uplink Gigabit Ethernet ports on the Supervisor I support a single receive queue with four thresholds (1q4t) and two transmit queues with two thresholds (2q2t). The uplink Gigabit Ethernet ports on the Supervisor IA and the Supervisor II cards include enhanced QoS features that provide an additional priority queue for both ingress (1p1q4t) and egress (1p2q2t) interfaces. These queues are serviced in a WRR method, except for the priority queue, which is always serviced as soon as frames have entered the queue. Table 10-7 lists the TX and RX queues supported by each supervisor engine.

Table 10-7 Supervior Queues and Thresholds

The addition of the priority queue on the Supervisor IA and Supervisor II offers an advantage for providing QoS for real-time applications over Supervisor I.

Cisco Catalyst Switch QoS Features 705

Ingress and egress scheduling are always based on the CoS value associated with the frame. By default, higher CoS values are mapped to higher queue numbers. CoS 5 traffic, typically associated with Voice over IP (VoIP) traffic, is mapped to the strict-priority queue, if present.

In addition to the different queues, each standard queue has one or more drop thresholds. There are two types of drop thresholds:

•Tail-drop thresholds

On ports with tail-drop thresholds, frames of a given CoS value are admitted to the queue until the drop threshold associated with that CoS value is exceeded; subsequent frames of that CoS value are discarded until the threshold is no longer exceeded.

If CoS 1 is assigned to Queue 1, threshold 2, for example, and the threshold 2 watermark is 60 percent, frames with CoS 1 are not dropped until Queue 1 is 60 percent full. All subsequent CoS 1 frames are dropped until the queue is less than 60 percent full.

•WRED-drop thresholds

On ports with Weighted Random Early Detection (WRED)-drop thresholds, frames of a given CoS value are admitted to the queue based on a random probability designed

to avoid buffer congestion. The probability of a frame with a given CoS being admitted to the queue or discarded depends on the weight and threshold assigned to that CoS value.

If CoS 2 is assigned to Queue 1, threshold 2, for example, and the threshold 2 watermarks are 40 percent (low) and 80 percent (high), frames with CoS 2 are not dropped until Queue 1 is at least 40 percent full. As the queue depth approaches 80 percent, frames with CoS 2 have an increasingly higher probability of being discarded rather than being admitted to the queue. When the queue is more than 80 percent full, all CoS 2 frames are dropped until the queue is less than 80 percent full. The frames that the switch discards when the queue level is between the low and high thresholds are picked at random, rather than on a per-flow or FIFO basis. This method works well with protocols such as TCP that are capable of adjusting to periodic packet drops by backing off and adjusting their transmission window size.

Enter the show qos info config [mod/port] command to determine whether your Catalyst switch port supports WRED. In Example 10-6, the show qos info config command shows that port 2/ 1 does not support WRED.

Example 10-6 The show qos info config Command

Cat6500> show qos info config 2/1

QoS setting in NVRAM:

QoS is enabled

Port 2/1 has 2 transmit queue with 2 drop thresholds (2q2t).

Port 2/1 has 1 receive queue with 4 drop thresholds (1q4t).

Interface type:vlan-based

ACL attached:

The qos trust type is set to untrusted.