Configuring Quality of Service

Note For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst 6000 and 6500 Series Command Reference publication.

This chapter consists of these sections:

Understanding How QoS Works

When you enable QoS (see the "Enabling QoS" section), the Catalyst 6000 and 6500 series switches provide QoS based on IEEE 802.1p class of service (CoS) values.

These sections describe 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.

QoS uses classification and scheduling to transmit network traffic from the switch in a predictable manner. QoS classifies frames by assigning priority-indexed CoS values to them and implements scheduling with receive queue and transmit queue drop thresholds that use the CoS values to give preference to higher-priority traffic. See Figure 9-1.

Figure 9-1: Traffic Flow Through the Switch with QoS Enabled

Understanding CoS Values

CoS values range between zero for low-priority and seven for high-priority.

Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries the CoS value in the three least significant bits. IEEE 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. Other frame types cannot carry CoS values.

Note On Catalyst 6000 and 6500 series switches, QoS does not read or set the IP precedence bits in the Layer 3 IP type of service (ToS) field.

Trusted and Untrusted Ports

Ports can be configured as either trusted or untrusted, indicating whether or not QoS should trust the CoS values in received frames to be consistent with network policy. On trusted ports, QoS accepts received CoS values. On untrusted ports, QoS classifies the frame with the port CoS value.

Understanding Classification

Classification is the assignment of CoS values to network traffic.

These sections describe classification:

Understanding Classification at a Trusted Port

ISL or 802.1Q frames received through trusted ports do not require classification.

When an ISL frame enters the switch through a trusted ingress port, QoS accepts the three least significant bits in the User field as a CoS value.
When an 802.1Q frame enters the switch through a trusted ingress port, QoS accepts the User Priority bits as a CoS value.

QoS classifies all other frame types that enter the switch through a trusted ingress port with the configured CoS value of the port (the default is zero).

Understanding Classification at an Untrusted Port

QoS classifies all frames that enter the switch through an untrusted port with the configured CoS value of the port (the default is zero).

Understanding Classification Based on Destination

After network traffic passes through the ingress port, QoS can rewrite the CoS value with a configurable value in frames destined for selected pairs of destination host Media Access Control (MAC) addresses and VLANs (see the "Mapping a CoS Value to a Host Destination MAC Address/VLAN Pair" section).

Note QoS applies destination-based CoS values to frames after the frames pass through the receive queue. Destination-based CoS values are not in place when frames encounter receive queue drop thresholds during ingress port scheduling.

Ingress Port Scheduling

QoS uses receive queue drop thresholds to schedule network traffic entering the switch through a trusted port. Each port on the switch has a single receive queue buffer for incoming traffic.

QoS does not implement scheduling on untrusted ports, since the CoS values may not be valid. If a port is untrusted and its receive queue buffer overflows, the switch drops overflow frames without regard to CoS values.

If a port is trusted, QoS implements four receive drop thresholds in the receive queue to schedule incoming traffic according to CoS values:
- Using receive queue drop threshold 1, the switch drops incoming frames with CoS 0 or 1 when the receive queue buffer is 50 percent or more full.
- Using receive queue drop threshold 2, the switch drops incoming frames with CoS 2 or 3 when the receive queue buffer is 60 percent or more full.
- Using receive queue drop threshold 3, the switch drops incoming frames with CoS 4 or 5 when the receive queue buffer is 80 percent or more full.
- Using receive queue drop threshold 4, the switch drops incoming frames with CoS 6 or 7 when the receive queue buffer is 100 percent full.

Note The explanations in this section use default values. You can configure the default port CoS value on each port (see the "Configuring the CoS Value for a Port" section). You can configure the receive queue drop threshold percentages (see the "Configuring Receive Queue Drop Thresholds" section). You can configure the CoS values mapped to each drop threshold (see the "Mapping CoS Values to Drop Thresholds" section). All ports in the switch use the same drop threshold configuration.

Egress Port Scheduling

QoS uses transmit queue drop thresholds to schedule transmission of network traffic from the switch. QoS configures each port with a low-priority transmit queue and a high-priority transmit queue. The default QoS configuration allocates 80 percent of the total transmit queue bandwidth to the low-priority queue and 20 percent to the high-priority queue. Each transmit queue has two drop thresholds that function as follows:

Frames with CoS 0, 1, 2, or 3 go to the low-priority transmit queue (queue 1).
- Using transmit queue 1 drop threshold 1, the switch drops frames with CoS 0 or 1 when the low-priority transmit queue buffer is 40 percent full.
- Using transmit queue 1 drop threshold 2, the switch drops frames with CoS 2 or 3 when the low-priority transmit queue buffer is 100 percent full.
Frames with CoS 4, 5, 6, or 7 go to the high-priority transmit queue (queue 2).
- Using transmit queue 2 drop threshold 1, the switch drops frames with CoS 4 or 5 when the high-priority transmit queue buffer is 40 percent full.
- Using transmit queue 2 drop threshold 2, the switch drops frames with CoS 6 or 7 when the high-priority transmit queue buffer is 100 percent full.

Note The explanations in this section use default values. You can configure the transmit drop threshold percentages (see the "Configuring Transmit Queue Drop Thresholds" section). You can configure how the low- and high-priority transmit queues share the total available transmit queue bandwidth (see the "Allocating Bandwidth Between Transmit Queues" section and the "Configuring the Transmit Queue Size Ratio" section). You can configure the CoS values mapped to each transmit queue and the CoS values mapped to each threshold (see the "Mapping CoS Values to Drop Thresholds" section). All ports in the switch use the same transmit queue and drop threshold configuration.

Note QoS writes the CoS value (which may be the received CoS value, the port CoS value, or a destination-based CoS value) into ISL and 802.1Q frames transmitted from the switch. Other frame types cannot carry CoS values.

QoS Default Configuration

Table 9-1 shows the QoS default configuration.

Table 9-1: QoS Default Configuration
Feature Default Value

QoS enable state

Disabled

Port trust state

Untrusted

Port CoS value

Zero

With QoS enabled

Receive queue drop threshold¹ percentages

Threshold 1: 50%

Threshold 2: 60%

Threshold 3: 80%

Threshold 4: 100%

Transmit queue drop threshold percentages

Low-priority queue threshold 1: 40%

Low-priority queue threshold 2: 100%

High-priority queue threshold 1: 40%

High-priority queue threshold 2: 100%

Transmit queue low-priority/high-priority bandwidth allocation ratio

100:255

Transmit queue size ratio

Low priority: 80%

High priority: 20%

CoS value/drop threshold mapping

Receive drop threshold 1 and transmit queue 1/drop threshold 1: CoS 0 and 1

Receive drop threshold 2 and transmit queue 1/drop threshold 2: CoS 2 and 3

Receive drop threshold 3 and transmit queue 2/drop threshold 1: CoS 4 and 5

Receive drop threshold 4 and transmit queue 2/drop threshold 2: CoS 6 and 7

With QoS disabled

Receive queue drop threshold¹ percentages

All thresholds set to 100%

Transmit queue drop threshold percentages

All thresholds set to 100%

Transmit queue low-priority/high-priority bandwidth allocation ratio

255:1

Transmit queue size ratio

Low priority: 100%

High priority: Not used

CoS value/drop threshold mapping

Receive drop threshold 1 and transmit queue 1/drop threshold 1: CoS 0-7

¹ QoS implements receive queue drop thresholds only on trusted ports.

**Table 9-1: QoS Default Configuration**
Feature	Default Value
QoS enable state	Disabled
Port trust state	Untrusted
Port CoS value	Zero
With QoS enabled
Receive queue drop threshold¹ percentages	Threshold 1: 50% Threshold 2: 60% Threshold 3: 80% Threshold 4: 100%
Transmit queue drop threshold percentages	Low-priority queue threshold 1: 40% Low-priority queue threshold 2: 100% High-priority queue threshold 1: 40% High-priority queue threshold 2: 100%
Transmit queue low-priority/high-priority bandwidth allocation ratio	100:255
Transmit queue size ratio	Low priority: 80% High priority: 20%
CoS value/drop threshold mapping	Receive drop threshold 1 and transmit queue 1/drop threshold 1: CoS 0 and 1 Receive drop threshold 2 and transmit queue 1/drop threshold 2: CoS 2 and 3 Receive drop threshold 3 and transmit queue 2/drop threshold 1: CoS 4 and 5 Receive drop threshold 4 and transmit queue 2/drop threshold 2: CoS 6 and 7
With QoS disabled
Receive queue drop threshold¹ percentages	All thresholds set to 100%
Transmit queue drop threshold percentages	All thresholds set to 100%
Transmit queue low-priority/high-priority bandwidth allocation ratio	255:1
Transmit queue size ratio	Low priority: 100% High priority: Not used
CoS value/drop threshold mapping	Receive drop threshold 1 and transmit queue 1/drop threshold 1: CoS 0-7

Configuring QoS

These sections describe how to configure QoS on the Catalyst 6000 and 6500 series switches:

Note Because some QoS commands disable and then reenable ports, which may cause Spanning-Tree Protocol (STP) reconfiguration, enter QoS commands only when necessary. The messages resulting from disabling and enabling ports and from STP reconfiguration are not shown in the examples.

Enabling QoS

To enable QoS, perform this task in privileged mode:

Task Command

Enable QoS on the switch.

set qos enable

Task	Command
Enable QoS on the switch.	set qos enable

This example shows how to enable QoS:

Console> (enable) set qos enable
QoS is enabled.
Console> (enable)

Configuring the Trusted State of a Port

This command configures the trusted state of a port, indicating whether or not the frames arriving at the port are trusted to carry a CoS value consistent with network policy. By default, all ports are untrusted.

To configure the trusted state of a port, perform this task in privileged mode:

Task Command

Set the trusted state of a port.

set port qos mod_num/port_num trust {untrusted | trust-cos}

Task	Command
Set the trusted state of a port.	set port qos mod_num/port_num trust {untrusted \| trust-cos}

This example shows how to configure the trusted state of port 1/1:

Console> (enable) set port qos 1/1 trust trust-cos
Port 1/1 qos set to trust-cos
Console> (enable)

Configuring the CoS Value for a Port

Unclassified frames from trusted ports and all frames from untrusted ports are assigned the CoS value specified with this command.

To set the CoS value for a port, perform this task in privileged mode:

Task Command

Set the CoS value for a port.

set port qos mod_num/port_num cos cos-value

Task	Command
Set the CoS value for a port.	set port qos mod_num/port_num cos cos-value

This example shows how to set CoS equal to 3 in all unclassified frames received through port 1/1:

Console> (enable) set port qos 1/1 cos 3 
Port 1/1 qos cos set to 3
Console> (enable)

To revert to the default CoS value for a port, perform this task in privileged mode:

Task Command

Revert to the CoS value for a port.

clear port qos mod_num/port_num cos

Task	Command
Revert to the CoS value for a port.	clear port qos mod_num/port_num cos

This example shows how to revert to the default CoS value for port 1/1:

Console> (enable) clear port qos 1/1 cos
Port 1/1 qos cos setting cleared.
Console> (enable)

Configuring Receive Queue Drop Thresholds

To configure the receive queue drop thresholds on all trusted ports in the switch, perform this task in privileged mode:

Task Command

Set the receive queue drop thresholds.

set qos drop-threshold 1q4t rx queue 1 thr1 thr2 thr3 thr4

Task	Command
Set the receive queue drop thresholds.	set qos drop-threshold 1q4t rx queue 1 thr1 thr2 thr3 thr4

The thresholds are all specified as percentages, ranging from 1 to 100. A value of 10 indicates a threshold when the buffer is 10 percent full.

This example shows how to configure the receive queue drop thresholds:

Console> (enable) set qos drop-threshold 1q4t rx queue 1 20 40 75 100
Receive drop thresholds for queue 1 set at 20% 40% 75% 100%
Console> (enable)

Mapping a CoS Value to a Host Destination MAC Address/VLAN Pair

To assign a CoS value to all frames destined for a particular host destination MAC address and VLAN number value pair, perform this task in privileged mode:

Task Command

Assign a CoS value to a host destination MAC address/VLAN pair.

set qos mac-cos dest_MAC_addr VLAN cos_value

Task	Command
Assign a CoS value to a host destination MAC address/VLAN pair.	set qos mac-cos dest_MAC_addr VLAN cos_value

This example shows how to assign CoS 2 to a destination MAC address and VLAN 525:

Console> (enable) set qos mac-cos 00-40-0b-30-03-48 525 2
CoS 2 is assigned to 00-40-0b-30-03-48 vlan 525.
Console> (enable)

Configuring Transmit Queue Drop Thresholds

To configure the transmit queue drop thresholds on all ports in the switch, perform this task in privileged mode:

Task Command

Set the transmit drop queue thresholds.

set qos drop-threshold 2q2t tx queue q# thr1 thr2

Task	Command
Set the transmit drop queue thresholds.	set qos drop-threshold 2q2t tx queue q# thr1 thr2

The queue number is 1 for the low-priority transmit queue and 2 for the high-priority queue.

The thresholds are all specified as percentages, ranging from 1 to 100. A value of 10 indicates a threshold when the buffer is 10 percent full.

This example shows how to configure the low-priority transmit queue drop thresholds:

Console> (enable) set qos drop-threshold 2q2t tx queue 1 40 100
Transmit drop thresholds for queue 1 set at 40%  100%
Console> (enable)

Allocating Bandwidth Between Transmit Queues

The switch transmits frames from one queue at a time using a weighted-round robin (WRR) algorithm. WRR uses a weight value to decide how much to transmit from one queue before switching to the other. The higher the weight assigned to a queue, the more transmit bandwidth is allocated to it.

To allocate bandwidth, perform this task in privileged mode:

Task Command

Allocate bandwidth between transmit queue 1 (low-priority) and transmit queue 2 (high-priority).

set qos wrr 2q2t queue1-weight queue2-weight

Task	Command
Allocate bandwidth between transmit queue 1 (low-priority) and transmit queue 2 (high-priority).	set qos wrr 2q2t queue1-weight queue2-weight

The valid values for weight range from 1-255. This example shows how to configure bandwidth:

Console> (enable) set qos wrr 2q2t 30 70
QoS wrr ratio is set successfully.
Console> (enable)

Configuring the Transmit Queue Size Ratio

Estimate the mix of low priority-to-high priority traffic on your network (for example, 80 percent low-priority traffic and 20 percent high-priority traffic). Specify queue ratios with the estimated percentages, which must range from 1 to 99 and together add up to 100.

To set the transmit queue size ratio, perform this task in privileged mode:

Task Command

Set the transmit queue size ratio between transmit queue 1 (low-priority) and transmit queue 2 (high-priority).

set qos txq-ratio 2q2t queue1-val queue2-val

Task	Command
Set the transmit queue size ratio between transmit queue 1 (low-priority) and transmit queue 2 (high-priority).	set qos txq-ratio 2q2t queue1-val queue2-val

This example shows how to set the transmit queue size ratio:

Console> (enable) set qos txq-ratio 2q2t 80 20
QoS txq-ratio is set successfully.
Console> (enable)

Mapping CoS Values to Drop Thresholds

This command associates CoS values with both receive and transmit drop thresholds.

To associate CoS values to a drop threshold, perform this task in privileged mode:

Task Command

Associate a CoS value to a drop threshold.

set qos map 2q2t q# thr# cos coslist

Task	Command
Associate a CoS value to a drop threshold.	set qos map 2q2t q# thr# cos coslist

The receive and transmit drop thresholds have this relationship:

Receive queue drop threshold 1 = transmit queue 1 (low priority) drop threshold 1
Receive queue drop threshold 2 = transmit queue 1 (low priority) drop threshold 2
Receive queue drop threshold 3 = transmit queue 2 (high priority) drop threshold 1
Receive queue drop threshold 4 = transmit queue 2 (high priority) drop threshold 2

Use the transmit queue and drop threshold values in this command. This example shows how to map the CoS values 0 and 1 to both receive queue drop threshold 1 and transmit queue 1/drop threshold 1:

Console> (enable) set qos map 2q2t 1 1 cos 0,1
Qos tx priority queue and threshold mapped to cos successfully.
Console> (enable)

To revert to default CoS value/drop threshold mapping, perform this task in privileged mode:

Task Command

Revert to QoS map defaults.

clear qos map 2q2t

Task	Command
Revert to QoS map defaults.	clear qos map 2q2t

This example shows how to revert to QoS map defaults:

Console> (enable) clear qos map 2q2t
Qos map setting cleared.
Console> (enable)

Displaying QoS Information

To display QoS information, perform this task:

Task Command

Show QoS information.

show qos info [runtime | config] mod_num/port_num

Task	Command
Show QoS information.	show qos info [runtime \| config] mod_num/port_num

The config keyword displays the QoS configuration.

The runtime keyword displays the QoS configuration in use. When QoS is disabled, the display with the runtime keyword is "QoS is disabled." When QoS is enabled, the display with the runtime keyword is similar to that of the config keyword, except that current port activity is also shown.

This example shows how to display the QoS runtime information for port 2/1:

Console> show qos info runtime 2/1
Run time setting of QoS:
QoS is enabled on 2/1
Port 2/1 has 2 transmit queue with 2 drop thresholds (2q2t).
Port 2/1 has 1 receive queue with 4 drop thresholds (1q4t).
The qos trust type is set to untrusted.
Default CoS = 0    
Queue and Threshold Mapping:
Queue Threshold CoS            
----- --------- ---------------
1     1         0 1 
1     2         2 3 
2     1         4 5 
2     2         6 7 
Rx drop thresholds:
Rx drop thresholds are disabled for untrusted ports.
Tx drop thresholds:
Queue #  Thresholds - percentage (abs values )
-------  -------------------------------------
1        40% (144224 bytes) 100% (360416 bytes) 
2        40% (32864 bytes) 100% (77792 bytes) 
Queue Sizes:
Queue #  Sizes - percentage (abs values )
-------  -------------------------------------
1        80% (360416 bytes)
2        20% (81888 bytes)
WRR Configuration:
Ports with speed 1000Mbps have ratio of 100:255 between transmit queue 1 and 2 (25600:65280 bytes)

To show QoS information about receive or transmit queues, perform this task:

Task Command

Show QoS information about receive or transmit queues.

show qos info config {1q4t rx | 2q2t tx}

Task	Command
Show QoS information about receive or transmit queues.	show qos info config {1q4t rx \| 2q2t tx}

Use the 1q4t and rx keywords for the receive queues (all receive queues on all ports use the same configuration).

Use the 2q2t and tx keywords for the transmit queues (all transmit queues on all ports use the same configuration).

This example shows how to show the QoS information for the transmit queues:

Console> show qos info config 2q2ttx
QoS setting in NVRAM for 2q2t transmit:
QoS is enabled
Queue and Threshold Mapping:
Queue Threshold CoS            
----- --------- ---------------
1     1         0 1 
1     2         2 3 
2     1         4 5 
2     2         6 7 
Tx drop thresholds:
Queue #  Thresholds - percentage (abs values )
-------  -------------------------------------
1        40% 100% 
2        40% 100% 
Queue Sizes:
Queue #  Sizes - percentage (abs values )
-------  -------------------------------------
1        80% 
2        20% 
WRR Configuration:
Ports with 2q2t have ratio of 100:255 between transmit queue 1 and 2 
Console>

Displaying QoS Statistics

To show QoS statistics, perform this task:

Task Command

Show QoS statistics.

show qos statistics [mod_num[/port_num]]

Task	Command
Show QoS statistics.	show qos statistics [mod_num[/port_num]]

This example shows how to display QoS statistics for port 2/1:

Console> show qos statistics 2/1
On Transmit:Port 2/1 has 2 Queue(s) 2 Threshold(s)
Q #  Threshold #:Packets dropped
---  -----------------------------------------------
1    1:0 pkts , 2:0 pkts 
2    1:0 pkts , 2:0 pkts 
On Receive:Port 2/1 has 1 Queue(s) 4 Threshold(s)
Rx drop threshold counters are disabled for untrusted ports.
Q #  Threshold #:Packets dropped
---  -----------------------------------------------
1    1:0 pkts, 2:0 pkts, 3:0 pkts, 4:0 pkts
Console>

Reverting to QoS Defaults

Note Reverting to defaults disables QoS, since QoS is disabled by default.

To revert to QoS defaults, perform this task in privileged mode:

Task Command

Revert to QoS defaults.

clear qos config

Task	Command
Revert to QoS defaults.	clear qos config

This example shows how to revert to QoS defaults:

Console> (enable) clear qos config
This command will disable QoS and take values back to factory default.
Do you want to continue (y/n) [n]? y
QoS config cleared.
Console> (enable)

Disabling QoS

To disable QoS, perform this task in privileged mode:

Task Command

Disable QoS on the switch.

set qos disable

Task	Command
Disable QoS on the switch.	set qos disable

This example shows how to disable QoS:

Console> (enable) set qos disable
QoS is disabled.
Console> (enable)

Table of Contents

Configuring Quality of Service