Cisco Systems OL-4266-08 manual Prioritizing Traffic on Interswitch Links, 42-117

Chapter 42 Configuring PFC QoS

Common QoS Scenarios

If you know that all traffic entering a particular port can be trusted (as is the case on access-distribution or distribution-core uplink ports), you can use the port trust configuration. Using port trust does not provide any support for different traffic types entering a port, but it is a much simpler configuration option. This is the command syntax for port trust:

interface gigabitethernet 5/1 mls qos trust dscp

With ports configured to trust received DSCP, the DSCP value for the traffic leaving the router will be the same as the DSCP value for the traffic entering the trusted ports. After you have configured the trust state, you can use the following commands to verify that the setting has taken effect:

Router# show queueing interface gigabitethernet 5/1 include Trust

Trust state:trust DSCP

Prioritizing Traffic on Interswitch Links

This section describes how the routers operate using trusted values.

One of the most fundamental principles of QoS is to protect high-priority traffic in the case of oversubscription. The marking and trusting actions described in the “Classifying Traffic from PCs and IP Phones in the Access Layer” section on page 42-113and the “Accepting the Traffic Priority Value on Interswitch Links” section on page 42-116prepare the traffic to handle oversubscription, but they do not provide different levels of service. To achieve differing levels of service, the networking device must have an advanced scheduling algorithm to prioritize traffic as it sends traffic from a particular interface. This scheduling function is responsible for transmitting the high-priority traffic with greater frequency than the low-priority traffic. The net effect is a differentiated service for the various traffic classes.

These two concepts are fundamental to the provision of differentiated service for various traffic classes:

•Assigning the traffic to a particular queue

•Setting the queue scheduling algorithm

Once QoS has been enabled, default values are applied for both of these features. For many networks, these default values are sufficient to differentiate the network traffic. For other networks, theses values might need to be adjusted to produce the desired result. Only in rare cases should there be a need for significant changes from the default settings for these features.

The Cisco 7600 series router Ethernet modules support a variety of queue structures, ranging from a single queue up to an eight-queue architecture. You can compare the queue structure to a group of traffic lanes used to service different traffic types. For example, the police get prioritized treatment when driving down the freeway so that they can get to accidents or crime scenes quickly. In an analogous way, the voice traffic on an IP network requires the same prioritized treatment. The switch uses the queue structure to provide these lanes of differentiated service.

The exact queue type is specific to the Ethernet module that you are working with. This example uses a module that has four transmit queues, described as 1p3q8t, which indicates:

•One strict priority queue (1p)

•Three regular queues supporting Weighted-Round Robin scheduling (3q), each with eight WRED thresholds (8t, not discussed here)

Cisco 7600 series router Ethernet modules also have input queue structures, but these are used less often, and because there probably will not be congestion within the switch fabric, this example does not include them.

	Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
OL-4266-08		42-117