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Catalyst 3750 Switch Software Configuration Guide, 12.2(25)SEE
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Index
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Preface
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Overview
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Using the Command-Line Interface
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Assigning the Switch IP Address and Default Gateway
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Configuring Cisco IOS CNS Agents
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Managing Switch Stacks
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Clustering Switches
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Administering the Switch
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Configuring SDM Templates
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Configuring Switch-Based Authentication
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Configuring IEEE 802.1x Port-Based Authentication
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Configuring Interface Characteristics
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Configuring Smartports Macros
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Configuring VLANs
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Configuring VTP
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Configuring Voice VLAN
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Configuring Private VLANs
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Configuring IEEE 802.1Q and Layer 2 Protocol Tunneling
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Configuring STP
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Configuring MSTP
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Configuring Optional Spanning-Tree Features
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Configuring Flex Links
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Configuring DHCP Features and IP Source Guard
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Configuring Dynamic ARP Inspection
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Configuring IGMP Snooping and MVR
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Configuring Port-Based Traffic Control
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Configuring CDP
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Configuring UDLD
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Configuring SPAN and RSPAN
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Configuring RMON
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Configuring System Message Logging
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Configuring SNMP
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Configuring Network Security with ACLs
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Configuring QoS
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Configuring EtherChannels
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Configuring IP Unicast Routing
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Configuring IPv6 Unicast Routing
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Configuring IPv6 MLD Snooping
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Configuring IPv6 ACLs
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Configuring HSRP
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Configuring IP Multicast Routing
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Configuring MSDP
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Configuring Fallback Bridging
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Troubleshooting
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Configuring Online Diagnostics
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Supported MIBs
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Working with the Cisco IOS File System, Configuration Files, and Software Images
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Unsupported Commands in Cisco IOS Release 12.2(25)SEE
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Table Of Contents
Classification Based on QoS ACLs
Classification Based on Class Maps and Policy Maps
Queueing and Scheduling Overview
Queueing and Scheduling on Ingress Queues
Queueing and Scheduling on Egress Queues
Generated Auto-QoS Configuration
Effects of Auto-QoS on the Configuration
Auto-QoS Configuration Guidelines
Upgrading from a Previous Software Release
Auto-QoS Configuration Example
Displaying Auto-QoS Information
Default Standard QoS Configuration
Default Ingress Queue Configuration
Default Egress Queue Configuration
Default Mapping Table Configuration
Standard QoS Configuration Guidelines
Enabling VLAN-Based QoS on Physical Ports
Configuring Classification Using Port Trust States
Configuring the Trust State on Ports within the QoS Domain
Configuring the CoS Value for an Interface
Configuring a Trusted Boundary to Ensure Port Security
Enabling DSCP Transparency Mode
Configuring the DSCP Trust State on a Port Bordering Another QoS Domain
Classifying Traffic by Using ACLs
Classifying Traffic by Using Class Maps
Classifying, Policing, and Marking Traffic on Physical Ports by Using Policy Maps
Classifying, Policing, and Marking Traffic on SVIs by Using Hierarchical Policy Maps
Classifying, Policing, and Marking Traffic by Using Aggregate Policers
Configuring the CoS-to-DSCP Map
Configuring the IP-Precedence-to-DSCP Map
Configuring the Policed-DSCP Map
Configuring the DSCP-to-CoS Map
Configuring the DSCP-to-DSCP-Mutation Map
Configuring Ingress Queue Characteristics
Mapping DSCP or CoS Values to an Ingress Queue and Setting WTD Thresholds
Allocating Buffer Space Between the Ingress Queues
Allocating Bandwidth Between the Ingress Queues
Configuring the Ingress Priority Queue
Configuring Egress Queue Characteristics
Allocating Buffer Space to and Setting WTD Thresholds for an Egress Queue-Set
Mapping DSCP or CoS Values to an Egress Queue and to a Threshold ID
Configuring SRR Shaped Weights on Egress Queues
Configuring SRR Shared Weights on Egress Queues
Configuring the Egress Expedite Queue
Limiting the Bandwidth on an Egress Interface
Displaying Standard QoS Information
Configuring QoS
This chapter describes how to configure quality of service (QoS) by using automatic QoS (auto-QoS) commands or by using standard QoS commands on the Catalyst 3750 switch. With QoS, you can provide preferential treatment to certain types of traffic at the expense of others. Without QoS, the switch offers best-effort service to each packet, regardless of the packet contents or size. It sends the packets without any assurance of reliability, delay bounds, or throughput. Unless otherwise noted, the term switch refers to a standalone switch and a switch stack.
In software releases earlier than Cisco IOS Release 12.2(25)SE, you can configure QoS only on physical ports. In Cisco IOS Release 12.2(25)SE or later, you can configure QoS on physical ports and on switch virtual interfaces (SVIs). Other than to apply policy maps, you configure the QoS settings, such as classification, queueing, and scheduling, the same way on physical ports and SVIs. When configuring QoS on a physical port, you apply a nonhierarchical policy map. When configuring QoS on an SVI, you apply a nonhierarchical or a hierarchical policy map. In the Catalyst 3750 Metro switch documentation, nonhierarchical policy maps are referred to as nonhierarchical single-level policy maps, and hierarchical policy maps are referred to as hierarchical dual-level policy maps.
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For complete syntax and usage information for the commands used in this chapter, see the command reference this release.
This chapter consists of these sections:
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The switch supports some of the modular QoS CLI (MQC) commands. For more information about the MQC commands, see the "Modular Quality of Service Command-Line Interface Overview" at this site:
Understanding QoS
Typically, networks operate on a best-effort delivery basis, which means that all traffic has equal priority and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an equal chance of being dropped.
When you configure the QoS feature, you can select specific network traffic, prioritize it according to its relative importance, and use congestion-management and congestion-avoidance techniques to provide preferential treatment. Implementing QoS in your network makes network performance more predictable and bandwidth utilization more effective.
The QoS implementation is based on the Differentiated Services (Diff-Serv) architecture, an emerging standard from the Internet Engineering Task Force (IETF). This architecture specifies that each packet is classified upon entry into the network.
The classification is carried in the IP packet header, using 6 bits from the deprecated IP type of service (ToS) field to carry the classification (class) information. Classification can also be carried in the Layer 2 frame. These special bits in the Layer 2 frame or a Layer 3 packet are described here and shown in Figure 33-1:
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Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries an IEEE 802.1p class of service (CoS) value in the three least-significant bits. On ports configured as Layer 2 ISL trunks, all traffic is in ISL frames.
Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the CoS value in the three most-significant bits, which are called the User Priority bits. On ports configured as Layer 2 802.1Q trunks, all traffic is in 802.1Q frames except for traffic in the native VLAN.
Other frame types cannot carry Layer 2 CoS values.
Layer 2 CoS values range from 0 for low priority to 7 for high priority.
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Layer 3 IP packets can carry either an IP precedence value or a Differentiated Services Code Point (DSCP) value. QoS supports the use of either value because DSCP values are backward-compatible with IP precedence values.
IP precedence values range from 0 to 7.
DSCP values range from 0 to 63.
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Figure 33-1 QoS Classification Layers in Frames and Packets
All switches and routers that access the Internet rely on the class information to provide the same forwarding treatment to packets with the same class information and different treatment to packets with different class information. The class information in the packet can be assigned by end hosts or by switches or routers along the way, based on a configured policy, detailed examination of the packet, or both. Detailed examination of the packet is expected to happen closer to the edge of the network so that the core switches and routers are not overloaded with this task.
Switches and routers along the path can use the class information to limit the amount of resources allocated per traffic class. The behavior of an individual device when handling traffic in the DiffServ architecture is called per-hop behavior. If all devices along a path provide a consistent per-hop behavior, you can construct an end-to-end QoS solution.
Implementing QoS in your network can be a simple or complex task and depends on the QoS features offered by your internetworking devices, the traffic types and patterns in your network, and the granularity of control that you need over incoming and outgoing traffic.
Basic QoS Model
To implement QoS, the switch must distinguish packets or flow from one another (classify), assign a label to indicate the given quality of service as the packets move through the switch, make the packets comply with the configured resource usage limits (police and mark), and provide different treatment (queue and schedule) in all situations where resource contention exists. The switch also needs to ensure that traffic sent from it meets a specific traffic profile (shape).
Figure 33-2 shows the basic QoS model. Actions at the ingress port include classifying traffic, policing, marking, queueing, and scheduling:
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Actions at the egress port include queueing and scheduling:
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Figure 33-2 Basic QoS Model
Classification
Classification is the process of distinguishing one kind of traffic from another by examining the fields in the packet. Classification is enabled only if QoS is globally enabled on the switch. By default, QoS is globally disabled, so no classification occurs.
During classification, the switch performs a lookup and assigns a QoS label to the packet. The QoS label identifies all QoS actions to be performed on the packet and from which queue the packet is sent.
The QoS label is based on the DSCP or the CoS value in the packet and decides the queueing and scheduling actions to perform on the packet. The label is mapped according to the trust setting and the packet type as shown in Figure 33-3.
You specify which fields in the frame or packet that you want to use to classify incoming traffic. For non-IP traffic, you have these classification options as shown in Figure 33-3:
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For IP traffic, you have these classification options as shown in Figure 33-3:
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For ports that are on the boundary between two QoS administrative domains, you can modify the DSCP to another value by using the configurable DSCP-to-DSCP-mutation map.
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For information on the maps described in this section, see the "Mapping Tables" section. For configuration information on port trust states, see the "Configuring Classification Using Port Trust States" section.
After classification, the packet is sent to the policing, marking, and the ingress queueing and scheduling stages.
Figure 33-3 Classification Flowchart
Classification Based on QoS ACLs
You can use IP standard, IP extended, or Layer 2 MAC ACLs to define a group of packets with the same characteristics (class). In the QoS context, the permit and deny actions in the access control entries (ACEs) have different meanings than with security ACLs:
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After a traffic class has been defined with the ACL, you can attach a policy to it. A policy might contain multiple classes with actions specified for each one of them. A policy might include commands to classify the class as a particular aggregate (for example, assign a DSCP) or rate-limit the class. This policy is then attached to a particular port on which it becomes effective.
You implement IP ACLs to classify IP traffic by using the access-list global configuration command; you implement Layer 2 MAC ACLs to classify non-IP traffic by using the mac access-list extended global configuration command. For configuration information, see the "Configuring a QoS Policy" section.
Classification Based on Class Maps and Policy Maps
A class map is a mechanism that you use to name a specific traffic flow (or class) and to isolate it from all other traffic. The class map defines the criteria used to match against a specific traffic flow to further classify it. The criteria can include matching the access group defined by the ACL or matching a specific list of DSCP or IP precedence values. If you have more than one type of traffic that you want to classify, you can create another class map and use a different name. After a packet is matched against the class-map criteria, you further classify it through the use of a policy map.
A policy map specifies which traffic class to act on. Actions can include trusting the CoS, DSCP, or IP precedence values in the traffic class; setting a specific DSCP or IP precedence value in the traffic class; or specifying the traffic bandwidth limitations and the action to take when the traffic is out of profile. Before a policy map can be effective, you must attach it to a port.
You create a class map by using the class-map global configuration command or the class policy-map configuration command. You should use the class-map command when the map is shared among many ports. When you enter the class-map command, the switch enters the class-map configuration mode. In this mode, you define the match criterion for the traffic by using the match class-map configuration command.
You create and name a policy map by using the policy-map global configuration command. When you enter this command, the switch enters the policy-map configuration mode. In this mode, you specify the actions to take on a specific traffic class by using the class, trust, or set policy-map configuration and policy-map class configuration commands.
The policy map can contain the police and police aggregate policy-map class configuration commands, which define the policer, the bandwidth limitations of the traffic, and the action to take if the limits are exceeded.
To enable the policy map, you attach it to a port by using the service-policy interface configuration command.
In software releases earlier than Cisco IOS Release 12.2(25)SE, you can apply a policy map only to a physical port. In Cisco IOS Release 12.2(25)SE or later, you can apply a nonhierarchical policy map to a physical port or an SVI. However, a hierarchical policy map can only be applied to an SVI. A hierarchical policy map contains two levels. The first level, the VLAN level, specifies the actions to be taken against a traffic flow on the SVI. The second level, the interface level, specifies the actions to be taken against the traffic on the physical ports that belong to the SVI. The interface-level actions are specified in the interface-level policy map.
For more information, see the "Policing and Marking" section. For configuration information, see the "Configuring a QoS Policy" section.
Policing and Marking
After a packet is classified and has a DSCP-based or CoS-based QoS label assigned to it, the policing and marking process can begin as shown in Figure 33-4.
Policing involves creating a policer that specifies the bandwidth limits for the traffic. Packets that exceed the limits are out of profile or nonconforming. Each policer decides on a packet-by-packet basis whether the packet is in or out of profile and specifies the actions on the packet. These actions, carried out by the marker, include passing through the packet without modification, dropping the packet, or modifying (marking down) the assigned DSCP of the packet and allowing the packet to pass through. The configurable policed-DSCP map provides the packet with a new DSCP-based QoS label. For information on the policed-DSCP map, see the "Mapping Tables" section. Marked-down packets use the same queues as the original QoS label to prevent packets in a flow from getting out of order.
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In software releases earlier than Cisco IOS Release 12.2(25)SE, you can configure policing only on a physical port. You can configure the trust state, set a new DSCP or IP precedence value in the packet, or define an individual or aggregate policer. For more information, see the "Policing on Physical Ports" section.
In Cisco IOS Release 12.2(25)SE or later, you can configure policing on a physical port or an SVI. For more information about configuring policing on physical ports, see the "Policing on Physical Ports" section. When configuring policy maps on an SVI, you can create a hierarchical policy map and can define an individual policer only in the secondary interface-level policy map. For more information, see the "Policing on SVIs" section.
After you configure the policy map and policing actions, attach the policy to an ingress port or SVI by using the service-policy interface configuration command. For configuration information, see the "Classifying, Policing, and Marking Traffic on Physical Ports by Using Policy Maps" section, the "Classifying, Policing, and Marking Traffic on SVIs by Using Hierarchical Policy Maps" section, and the "Classifying, Policing, and Marking Traffic by Using Aggregate Policers" section.
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Policing on Physical Ports
In policy maps on physical ports, you can create these types of policers:
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Policing uses a token-bucket algorithm. As each frame is received by the switch, a token is added to the bucket. The bucket has a hole in it and leaks at a rate that you specify as the average traffic rate in bits per second. Each time a token is added to the bucket, the switch verifies that there is enough room in the bucket. If there is not enough room, the packet is marked as nonconforming, and the specified policer action is taken (dropped or marked down).
How quickly the bucket fills is a function of the bucket depth (burst-byte), the rate at which the tokens are removed (rate-bps), and the duration of the burst above the average rate. The size of the bucket imposes an upper limit on the burst length and limits the number of frames that can be transmitted back-to-back. If the burst is short, the bucket does not overflow, and no action is taken against the traffic flow. However, if a burst is long and at a higher rate, the bucket overflows, and the policing actions are taken against the frames in that burst.
You configure the bucket depth (the maximum burst that is tolerated before the bucket overflows) by using the burst-byte option of the police policy-map class configuration command or the mls qos aggregate-policer global configuration command. You configure how fast (the average rate) that the tokens are removed from the bucket by using the rate-bps option of the police policy-map class configuration command or the mls qos aggregate-policer global configuration command.
Figure 33-4 shows the policing and marking process when these types of policy maps are configured:
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Figure 33-4 Policing and Marking Flowchart on Physical Ports
Policing on SVIs
Note
A hierarchical policy map has two levels. The first level, the VLAN level, specifies the actions to be taken against a traffic flow on an SVI. The second level, the interface level, specifies the actions to be taken against the traffic on the physical ports that belong to the SVI and are specified in the interface-level policy map.
When configuring policing on an SVI, you can create and configure a hierarchical policy map with these two levels:
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See the "Classifying, Policing, and Marking Traffic on SVIs by Using Hierarchical Policy Maps" section for an example of a hierarchical policy map.
Figure 33-5 shows the policing and marking process when hierarchical policy maps on an SVI.
Figure 33-5 Policing and Marking Flowchart on SVIs
Mapping Tables
During QoS processing, the switch represents the priority of all traffic (including non-IP traffic) with an QoS label based on the DSCP or CoS value from the classification stage:
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On an ingress port configured in the DSCP-trusted state, if the DSCP values are different between the QoS domains, you can apply the configurable DSCP-to-DSCP-mutation map to the port that is on the boundary between the two QoS domains. You configure this map by using the mls qos map dscp-mutation global configuration command.
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The CoS-to-DSCP, DSCP-to-CoS, and the IP-precedence-to-DSCP maps have default values that might or might not be appropriate for your network.
The default DSCP-to-DSCP-mutation map and the default policed-DSCP map are null maps; they map an incoming DSCP value to the same DSCP value. The DSCP-to-DSCP-mutation map is the only map you apply to a specific port. All other maps apply to the entire switch.
For configuration information, see the "Configuring DSCP Maps" section.
For information about the DSCP and CoS input queue threshold maps, see the "Queueing and Scheduling on Ingress Queues" section. For information about the DSCP and CoS output queue threshold maps, see the "Queueing and Scheduling on Egress Queues" section.
Queueing and Scheduling Overview
The switch has queues at specific points to help prevent congestion as shown in Figure 33-6.
Figure 33-6 Ingress and Egress Queue Location
Because the total inbound bandwidth of all ports can exceed the bandwidth of the stack ring, ingress queues are located after the packet is classified, policed, and marked and before packets are forwarded into the switch fabric. Because multiple ingress ports can simultaneously send packets to an egress port and cause congestion, outbound queues are located after the stack ring.
Weighted Tail Drop
Both the ingress and egress queues use an enhanced version of the tail-drop congestion-avoidance mechanism called weighted tail drop (WTD). WTD is implemented on queues to manage the queue lengths and to provide drop precedences for different traffic classifications.
As a frame is enqueued to a particular queue, WTD uses the frame's assigned QoS label to subject it to different thresholds. If the threshold is exceeded for that QoS label (the space available in the destination queue is less than the size of the frame), the switch drops the frame.
Figure 33-7 shows an example of WTD operating on a queue whose size is 1000 frames. Three drop percentages are configured: 40 percent (400 frames), 60 percent (600 frames), and 100 percent (1000 frames). These percentages mean that up to 400 frames can be queued at the 40-percent threshold, up to 600 frames at the 60-percent threshold, and up to 1000 frames at the 100-percent threshold.
In this example, CoS values 6 and 7 have a greater importance than the other CoS values, and they are assigned to the 100-percent drop threshold (queue-full state). CoS values 4 and 5 are assigned to the 60-percent threshold, and CoS values 0 to 3 are assigned to the 40-percent threshold.
Suppose the queue is already filled with 600 frames, and a new frame arrives. It contains CoS values 4 and 5 and is subjected to the 60-percent threshold. If this frame is added to the queue, the threshold will be exceeded, so the switch drops it.
Figure 33-7 WTD and Queue Operation
For more information, see the "Mapping DSCP or CoS Values to an Ingress Queue and Setting WTD Thresholds" section, the "Allocating Buffer Space to and Setting WTD Thresholds for an Egress Queue-Set" section, and the "Mapping DSCP or CoS Values to an Egress Queue and to a Threshold ID" section.
SRR Shaping and Sharing
Both the ingress and egress queues are serviced by SRR, which controls the rate at which packets are sent. On the ingress queues, SRR sends packets to the stack ring. On the egress queues, SRR sends packets to the egress port.
You can configure SRR on egress queues for sharing or for shaping. However, for ingress queues, sharing is the default mode, and it is the only mode supported.
In shaped mode, the egress queues are guaranteed a percentage of the bandwidth, and they are rate-limited to that amount. Shaped traffic does not use more than the allocated bandwidth even if the link is idle. Shaping provides a more even flow of traffic over time and reduces the peaks and valleys of bursty traffic. With shaping, the absolute value of each weight is used to compute the bandwidth available for the queues.
In shared mode, the queues share the bandwidth among them according to the configured weights. The bandwidth is guaranteed at this level but not limited to it. For example, if a queue is empty and no longer requires a share of the link, the remaining queues can expand into the unused bandwidth and share it among them. With sharing, the ratio of the weights controls the frequency of dequeuing; the absolute values are meaningless.
For more information, see the "Allocating Bandwidth Between the Ingress Queues" section, the "Configuring SRR Shaped Weights on Egress Queues" section, and the "Configuring SRR Shared Weights on Egress Queues" section.
Queueing and Scheduling on Ingress Queues
Figure 33-8 shows the queueing and scheduling flowchart for ingress ports.
Figure 33-8 Queueing and Scheduling Flowchart for Ingress Ports
Note
The switch supports two configurable ingress queues, which are serviced by SRR in shared mode only. Table 33-1 describes the queues.
Table 33-1 Ingress Queue Types
Normal
User traffic that is considered to be normal priority. You can configure three different thresholds to differentiate among the flows. You can use the mls qos srr-queue input threshold, the mls qos srr-queue input dscp-map, and the mls qos srr-queue input cos-map global configuration commands.
Expedite
High-priority user traffic such as differentiated services (DF) expedited forwarding or voice traffic. You can configure the bandwidth required for this traffic as a percentage of the total stack traffic by using the mls qos srr-queue input priority-queue global configuration command. The expedite queue has guaranteed bandwidth.
1 The switch uses two nonconfigurable queues for traffic that is essential for proper network and stack operation.
You assign each packet that flows through the switch to a queue and to a threshold. Specifically, you map DSCP or CoS values to an ingress queue and map DSCP or CoS values to a threshold ID. You use the mls qos srr-queue input dscp-map queue queue-id {dscp1...dscp8 | threshold threshold-id dscp1...dscp8} or the mls qos srr-queue input cos-map queue queue-id {cos1...cos8 | threshold threshold-id cos1...cos8} global configuration command. You can display the DSCP input queue threshold map and the CoS input queue threshold map by using the show mls qos maps privileged EXEC command.
WTD Thresholds
The queues use WTD to support distinct drop percentages for different traffic classes. Each queue has three drop thresholds: two configurable (explicit) WTD thresholds and one nonconfigurable (implicit) threshold preset to the queue-full state. You assign the two explicit WTD threshold percentages for threshold ID 1 and ID 2 to the ingress queues by using the mls qos srr-queue input threshold queue-id threshold-percentage1 threshold-percentage2 global configuration command. Each threshold value is a percentage of the total number of allocated buffers for the queue. The drop threshold for threshold ID 3 is preset to the queue-full state, and you cannot modify it. For more information about how WTD works, see the "Weighted Tail Drop" section.
Buffer and Bandwidth Allocation
You define the ratio (allocate the amount of space) with which to divide the ingress buffers between the two queues by using the mls qos srr-queue input buffers percentage1 percentage2 global configuration command. The buffer allocation together with the bandwidth allocation control how much data can be buffered and sent before packets are dropped. You allocate bandwidth as a percentage by using the mls qos srr-queue input bandwidth weight1 weight2 global configuration command. The ratio of the weights is the ratio of the frequency in which the SRR scheduler sends packets from each queue.
Priority Queueing
You can configure one ingress queue as the priority queue by using the mls qos srr-queue input priority-queue queue-id bandwidth weight global configuration command. The priority queue should be used for traffic (such as voice) that requires guaranteed delivery because this queue is guaranteed part of the bandwidth regardless of the load on the stack ring.
SRR services the priority queue for its configured weight as specified by the bandwidth keyword in the mls qos srr-queue input priority-queue queue-id bandwidth weight global configuration command. Then, SRR shares the remaining bandwidth with both ingress queues and services them as specified by the weights configured with the mls qos srr-queue input bandwidth weight1 weight2 global configuration command.
You can combine the commands described in this section to prioritize traffic by placing packets with particular DSCPs or CoSs into certain queues, by allocating a large queue size or by servicing the queue more frequently, and by adjusting queue thresholds so that packets with lower priorities are dropped. For configuration information, see the "Configuring Ingress Queue Characteristics" section.
Queueing and Scheduling on Egress Queues
Figure 33-9 shows the queueing and scheduling flowchart for egress ports.
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Figure 33-9 Queueing and Scheduling Flowchart for Egress Ports
Each port supports four egress queues, one of which (queue 1) can be the egress expedite queue. These queues are assigned to a queue-set. All traffic exiting the switch flows through one of these four queues and is subjected to a threshold based on the QoS label assigned to the packet.
Figure 33-10 shows the egress queue buffer. The buffer space is divided between the common pool and the reserved pool. The switch uses a buffer allocation scheme to reserve a minimum amount of buffers for each egress queue, to prevent any queue or port from consuming all the buffers and depriving other queues, and to control whether to grant buffer space to a requesting queue. The switch detects whether the target queue has not consumed more buffers than its reserved amount (under-limit), whether it has consumed all of its maximum buffers (over limit), and whether the common pool is empty (no free buffers) or not empty (free buffers). If the queue is not over-limit, the switch can allocate buffer space from the reserved pool or from the common pool (if it is not empty). If there are no free buffers in the common pool or if the queue is over-limit, the switch drops the frame.
Figure 33-10 Egress Queue Buffer Allocation
Buffer and Memory Allocation
You guarantee the availability of buffers, set drop thresholds, and configure the maximum memory allocation for a queue-set by using the mls qos queue-set output qset-id threshold queue-id drop-threshold1 drop-threshold2 reserved-threshold maximum-threshold global configuration command. Each threshold value is a percentage of the queue's allocated memory, which you specify by using the mls qos queue-set output qset-id buffers allocation1 ... allocation4 global configuration command. The sum of all the allocated buffers represents the reserved pool, and the remaining buffers are part of the common pool.
Through buffer allocation, you can ensure that high-priority traffic is buffered. For example, if the buffer space is 400, you can allocate 70 percent of it to queue 1 and 10 percent to queues 2 through 4. Queue 1 then has 280 buffers allocated to it, and queues 2 through 4 each have 40 buffers allocated to them.
You can guarantee that the allocated buffers are reserved for a specific queue in a queue-set. For example, if there are 100 buffers for a queue, you can reserve 50 percent (50 buffers). The switch returns the remaining 50 buffers to the common pool. You also can enable a queue in the full condition to obtain more buffers than are reserved for it by setting a maximum threshold. The switch can allocate the needed buffers from the common pool if the common pool is not empty.
WTD Thresholds
You can assign each packet that flows through the switch to a queue and to a threshold. Specifically, you map DSCP or CoS values to an egress queue and map DSCP or CoS values to a threshold ID. You use the mls qos srr-queue output dscp-map queue queue-id {dscp1...dscp8 | threshold threshold-id dscp1...dscp8} or the mls qos srr-queue output cos-map queue queue-id {cos1...cos8 | threshold threshold-id cos1...cos8} global configuration command. You can display the DSCP output queue threshold map and the CoS output queue threshold map by using the show mls qos maps privileged EXEC command.
The queues use WTD to support distinct drop percentages for different traffic classes. Each queue has three drop thresholds: two configurable (explicit) WTD thresholds and one nonconfigurable (implicit) threshold preset to the queue-full state. You assign the two WTD threshold percentages for threshold ID 1 and ID 2. The drop threshold for threshold ID 3 is preset to the queue-full state, and you cannot modify it. For more information about how WTD works, see the "Weighted Tail Drop" section.
Shaped or Shared Mode
SRR services each queue-set in shared or shaped mode. You map a port to a queue-set by using the queue-set qset-id interface configuration command. You assign shared or shaped weights to the port by using the srr-queue bandwidth share weight1 weight2 weight3 weight4 or the srr-queue bandwidth shape weight1 weight2 weight3 weight4 interface configuration command. For an explanation of the differences between shaping and sharing, see the "SRR Shaping and Sharing" section.
Note
The buffer allocation together with the SRR weight ratios control how much data can be buffered and sent before packets are dropped. The weight ratio is the ratio of the frequency in which the SRR scheduler sends packets from each queue.
All four queues participate in the SRR unless the expedite queue is enabled, in which case the first bandwidth weight is ignored and is not used in the ratio calculation. The expedite queue is a priority queue, and it is serviced until empty before the other queues are serviced. You enable the expedite queue by using the priority-queue out interface configuration command.
You can combine the commands described in this section to prioritize traffic by placing packets with particular DSCPs or CoSs into certain queues, by allocating a large queue size or by servicing the queue more frequently, and by adjusting queue thresholds so that packets with lower priorities are dropped. For configuration information, see the "Configuring Egress Queue Characteristics" section.
Note
Packet Modification
A packet is classified, policed, and queued to provide QoS. Packet modifications can occur during this process:
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The input mutation causes the DSCP to be rewritten depending on the new value of DSCP chosen. The set action in a policy map also causes the DSCP to be rewritten.
Configuring Auto-QoS
You can use the auto-QoS feature to simplify the deployment of existing QoS features. Auto-QoS makes assumptions about the network design, and as a result, the switch can prioritize different traffic flows and appropriately use the ingress and egress queues instead of using the default QoS behavior. (The default is that QoS is disabled. The switch then offers best-effort service to each packet, regardless of the packet contents or size, and sends it from a single queue.)
When you enable auto-QoS, it automatically classifies traffic based on the traffic type and ingress packet label. The switch uses the resulting classification to choose the appropriate egress queue.
You use auto-QoS commands to identify ports connected to Cisco IP Phones and to devices running the Cisco SoftPhone application. You also use the commands to identify ports that receive trusted traffic through an uplink. Auto-QoS then performs these functions:
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These sections contain this configuration information:
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Generated Auto-QoS Configuration
By default, auto-QoS is disabled on all ports.
When auto-QoS is enabled, it uses the ingress packet label to categorize traffic, to assign packet labels, and to configure the ingress and egress queues as shown in Table 33-2.
Table 33-2 Traffic Types, Packet Labels, and Queues
TrafficDSCP
46
24, 26
48
56
34
-
CoS
5
3
6
7
4
-
CoS-to-Ingress Queue Map
2, 3, 4, 5, 6, 7 (queue 2)
0, 1 (queue 1)
CoS-to-Egress Queue Map
5 (queue 1)
3, 6, 7 (queue 2)
4 (queue 3)
2 (queue 3)
0, 1
(queue 4)
1 VoIP = voice over IP
Table 33-3 shows the generated auto-QoS configuration for the ingress queues.
Table 33-4 shows the generated auto-QoS configuration for the egress queues.
When you enable the auto-QoS feature on the first port, these automatic actions occur:
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For information about the trusted boundary feature, see the "Configuring a Trusted Boundary to Ensure Port Security" section.
When you enable auto-QoS by using the auto qos voip cisco-phone, the auto qos voip cisco-softphone, or the auto qos voip trust interface configuration command, the switch automatically generates a QoS configuration based on the traffic type and ingress packet label and applies the commands listed in Table 33-5 to the port.
Effects of Auto-QoS on the Configuration
When auto-QoS is enabled, the auto qos voip interface configuration command and the generated configuration are added to the running configuration.
The switch applies the auto-QoS-generated commands as if the commands were entered from the CLI. An existing user configuration can cause the application of the generated commands to fail or to be overridden by the generated commands. These actions occur without warning. If all the generated commands are successfully applied, any user-entered configuration that was not overridden remains in the running configuration. Any user-entered configuration that was overridden can be retrieved by reloading the switch without saving the current configuration to memory. If the generated commands fail to be applied, the previous running configuration is restored.
Auto-QoS Configuration Guidelines
Before configuring auto-QoS, you should be aware of this information:
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