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A primary differentiator between MQC on the Cisco Catalyst 6500 and router Cisco IOS Software is in the application of specific QoS features.
The Cisco Catalyst 6500 implements much of its QoS functionality in hardware. Router Cisco IOS Software differs in that it primarily implements
most features in software. For this reason, there are differences in how certain features are configured. More importantly, there are some QoS
features that are found in router Cisco IOS Software that are not found in the Cisco Catalyst 6500 hardware.
Ingress QoS: Trust
A packet can arrive at an interface with a priority value already present in the packets header. A question then arises for the switch: Is this priority
setting valid? Was it set by a valid application or network device according to predefined rules? Or maybe this priority setting was set by a user
hoping to get better service from the network? Either way, the switch has to make that determination and decide whether to honor the priority
valueµ or change it to another value. How the switch makes this determination is by using the port "trust" setting. (See Figure 5.)
Figure 5. Switch Port Trust Settings
When QoS is enabled on a switch, by default, all ports are placed into a state of untrusted. In this mode, any packet with an existing priority setting
that is received on an untrusted port will have its priority setting reset to a default CoS value (the default CoS value is zero). It is the responsibility
of the network administrator to identify ports where the priority value should be honored. For instance, connections to selected servers, IP
telephones, and IP call managers are examples of ports that should be set to trust the incoming priority setting.
When setting trust, the switch port can be set to trust one of the three priority settings, CoS, IP precedence, or DSCP. If a port is going to be set to
trust the incoming priority value, the network administrator has to make the determination of which of the three priority settings will be trusted.
Ingress and Egress QoS: Switch Port Queues
All line cards in the Cisco Catalyst 6500 provide a fixed set of ingress (also known as input or receive) queues and egress (also known as output or
transmit) queues per port. The number of queues is fixed in hardware on the line card and cannot be changed. Associated with the queue is
an amount of buffer memory that the queue uses to temporarily hold transient data. On some line cards, there is a dedicated amount of memory
available to each port. On other line cards, a pool of memory is shared between a set group of ports. A table later in this paper provides a summary
list of line cards, the queue structures on each, and the amount of buffering available to each port on the line card.
On select line cards, a strict priority queue is made available. This special queue can be used for latency-sensitive traffic and is designed to forward
data immediately when a packet arrives in that queue. More information on the strict priority queue is detailed later in this paper.
Ingress and Egress QoS: Thresholds
Thresholds have multiple uses within a queue. They are used within a queue to identify when the memory buffers have reached a certain predefined
utilization. After the threshold has been exceeded, the switch port will initiate a process to start dropping packets from the queue. One of two
mechanisms—tail drop and WRED—is typically used to perform this duty. Both of these processes are described later in this paper.
© 2005 Cisco Systems, Inc. All rights reserved.
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