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Cisco catalyst x6524: supplementary guide.
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Quality of Service on Cisco Catalyst 6500
INTRODUCTION
Picture a busy bank branch. The queue of customers lining up to see the tellers is very long. When you arrive at the bank branch, you are
immediately recognized as one of the bank's valued customers. You are escorted to a separate queue for "special" customers away from the hustle
and bustle of the long queue. In this special queue you get served by the teller immediately. As a special customer, you have just been given
a preferential level of service that exceeds what the average customer receives.
®
®
Like in the bank example above, quality of service (QoS) in Cisco
Catalyst
6500 Family line cards is a tool that is used to provide preferential
service for select traffic as it transits the switch. Over time and with the advancements in hardware and software technology, a number of QoS tools
have now become available. QoS in itself is not one feature, but a collection of features that when combined provide a powerful way to identify
different classes of traffic, prioritize them, and then service that traffic ahead of other lower priority traffic entering and leaving the switch.
This document will attempt to provide a high level overview of the QoS features found on the Cisco Catalyst 6500. It will explain what the features
are, how they work, and where in hardware they are performed.
WHERE IS QoS PERFORMED?
The Cisco Catalyst 6500 performs QoS on the supervisor and the line card. The supervisor contains a daughter card called the policy feature card
(PFC). Although the PFC is primarily responsible for the hardware forwarding of packets, it also performs a number of important QoS tasks. Since
the Cisco Catalyst 6500 was introduced in 1999, a number of PFC models aligned with specific supervisor models have been introduced. Typically
the introduction of a new PFC coincides with the arrival of new QoS features. This is especially true of the PFC3, which when introduced with the
Cisco Catalyst 6500 Series Supervisor Engine 720 in 2003 added a number of new QoS features not found in earlier PFC models.
The Cisco Catalyst 6500 line card is the other component that performs QoS, and those QoS features are primarily influenced by the port
application-specific integrated circuit (ASIC). The level of QoS support on the line card is dependent on the functionality built into the line-card port
ASIC. For this reason, the QoS capabilities can differ between different generations of the Cisco Catalyst 6500 Family.
Toward the end of this paper, a set of tables give an overview of the QoS features available on each of the line cards and PFC versions.
A QUICK RECAP ON SETTING PRIORITY
When data is sent through a network, it can be tagged with a priority value. When the data passes through a network device, the priority value is
used by that network device to determine how it should treat the packet. Data can be tagged with a priority value as described in the following
sections.
Class of Service
When a packet is transmitted out an Ethernet port, it has an Ethernet header attached to it. This Ethernet header can include an optional VLAN tag
(also referred to as an IEEE 802.1Q VLAN tag). Within the VLAN tag is a 3-bit field called the class-of-service (CoS) field. These 3 bits can be
manipulated to yield eight different priority values. Figure 1 shows where in the Ethernet header the priority bits are found.
All contents are Copyright © 1992–2005 Cisco Systems, Inc. All rights reserved. Important Notices and Privacy Statement.
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Table of Contents

   Also See for Cisco Catalyst X6524

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   Summary of Contents for Cisco Catalyst X6524

  • Page 1 This document will attempt to provide a high level overview of the QoS features found on the Cisco Catalyst 6500. It will explain what the features are, how they work, and where in hardware they are performed.
  • Page 2 Cisco Systems supports the Inter-Switch Link (ISL) VLAN tagging option on selected Cisco Catalyst 6500 line cards. ISL is a Cisco proprietary VLAN tagging option that also supports an inbuilt 3-bit CoS field just like the IEEE 802.1Q option mentioned earlier.
  • Page 3 Built into the header is the priority value that can be used to indicate to switches and routers in the network path of the priority of that piece of data. The Cisco Catalyst 6500 has the ability to change that priority value (increase or decrease it) if required based on any policies that may be set by the network administrator.
  • Page 4 THE QoS FEATURE TOOLKIT The Cisco Catalyst 6500 is primed with a number of QoS features that when combined provide an effective vehicle to better service higher priority traffic. The following section will attempt to present a high-level overview of the major QoS features in the Cisco Catalyst 6500.
  • Page 5 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.
  • Page 6 When a packet arrives at a switch port, it can be primed with a priority value. The trust setting of the port will determine which priority setting (ToS or CoS) will be honored by the switch. As the packet passes through the switch (that is, after it has arrived on the input port and prior to it being sent out the output port), it is assigned a priority value that is only used internal to the switch.
  • Page 7 When a packet arrives at a switch port, the trust setting of that port will derive an internal priority value (known as internal DSCP, as described earlier) that it uses to assign service to the packet while it transits the switch. When the packet is transmitted out a switch port, the actual DSCP value (written into the IP header) in the outgoing packet is derived from the internal DSCP value.
  • Page 8 The microflow policer differs slightly from an aggregate in that it applies a rate-limiting policy to each discrete flow. The question then becomes: What is a flow, and how is it defined by the Cisco Catalyst 6500? A flow is defined as a unidirectional flow of data that is uniquely identified by primary fields in the packet’s IP and TCP/User Datagram Protocol (UDP) headers.
  • Page 9 Congestion Avoidance: Tail Drop As a switch port queue begins to fill with data, thresholds can be used to identify what traffic can be dropped when the threshold is breached. A packet is primed with a priority value, and the priority value identifies with which threshold this packet is going to be associated. When that threshold is breached, any packet arriving at the queue with that priority value will be dropped.
  • Page 10 Scheduling: WRR With multiple queues on each switch port, WRR (see Figure 11) provides a way to schedule and send data held in those queues onto the wire. Used on the egress port, the configuration of WRR allows a weighting to be assigned to each queue, which is then used to determine the amount of bandwidth to which each queue has access.
  • Page 11 Scheduling: SRR SRR is a recent addition to the scheduling capabilities of the Cisco Catalyst 6500 Family. At the time of writing this paper, support for SRR is only available on the uplink ports of the Cisco Catalyst 6500 Series Supervisor Engine 32. SRR is different from WRR in that the SRR algorithm provides a way to shape outbound traffic to a stated rate.
  • Page 12 The concept of auto-QoS was designed to simplify the configuration and implementation of specific QoS features on the Cisco Catalyst 6500. Auto- QoS puts in place a set of macros that can be implemented from the Cisco Catalyst 6500 command line. In its initial implementation, the application of auto-QoS is focused on setting the QoS configuration for a given port to support an attached Cisco IP Phone.
  • Page 13 Auto-QoS is applied in two steps. The first step is to enable it at a system level. This sets in place a series of defaults, which will become active when a given port has auto-QoS applied. For each port that has an attached Cisco IP Phone, an auto-QoS macro is applied that sets in place a number of port-specific QoS settings specific for the device telephony type.
  • Page 14 LINE-CARD QoS SUMMARY Table 3 provides a summary of the QoS capabilities for each of the line cards in the Cisco Catalyst 6500 Family. Table 3. QoS Capability Summary for Line Cards xCEF720 Modules Description WS-X6704-10GE Cisco Catalyst 6500 4-port 10-GbE...
  • Page 15 Cisco Catalyst 6500 48-port fabric- enabled 10/100/1000 module WS-X6816-GBIC Cisco Catalyst 6500 16-port GigE mod: fabric-enabled (req. GBICs) Important notices, privacy statements, and trademarks of Cisco Systems, Inc. can be found on cisco.com. Receive Queue Structure 1q4t 1q4t 1q4t 1q4t...
  • Page 16 As a collection of technologies, distinct QoS features can be applied in different combinations to solve an assortment of problems in a network. The richness of the QoS feature set on the Cisco Catalyst 6500 provides network managers with a means to facilitate convergence and prioritize mission-critical application data.
  • Page 17 Copyright  2005 Cisco Systems, Inc. All rights reserved. CCSP, CCVP, the Cisco Square Bridge logo, Follow Me Browsing, and StackWise are trademarks of Cisco Systems, Inc.; Changing the Way We Work, Live, Play, and Learn, and iQuick Study are service marks of Cisco Systems, Inc.; and Access Registrar, Aironet, ASIST, BPX, Catalyst, CCDA, CCDP,...
  • Page 18 © 2005 Cisco Systems, Inc. All rights reserved. Important notices, privacy statements, and trademarks of Cisco Systems, Inc. can be found on cisco.com. Page 18 of 18...

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