Class of Service (CoS) Configuration

Contents

Introduction

Given that there will always be points in the network where multiple traffic streams merge or where network links will change speed and capacity, it is important to move traffic on the basis of relative importance. Without CoS prioritization, less important traffic can consume network bandwidth and slow down or halt the delivery of more important traffic. For example, without CoS, most traffic received by a switch is forwarded with the same priority it had upon entering the switch. In many cases, such traffic is ‘‘normal’’ priority and competes for bandwidth with all other normal-priority traffic, regardless of its relative importance to your organization’s mission. CoS helps to keep the most important network traffic moving at an acceptable speed, regardless of current bandwidth usage. This means you can manage available bandwidth so that the switch transmits the most important traffic first.

Using VLANs to Maximize the CoS Benefit

Without an 802.1Q tagged VLAN environment, CoS can prioritize only the movement of outbound traffic through individual ProCurve switches in your network. Using 802.1Q tagged VLANs provides the maximum advantage by allowing CoS to set priorities that are supported by downstream devices. This means that CoS can support improved performance for present traffic levels as well as future traffic growth while optimizing the use of existing resources and delaying the need for further investments in equipment and services. That is, CoS enables you to:

Definitions

Term Definition
differentiated
services bits
The upper 6 bits of the Type of Service (ToS) field (the DS field) of an IP packet.
downstream device A device linked directly or indirectly to an outbound switch port. That is, the switch sends traffic to downstream devices.
inbound port Any port on a switch through which traffic enters the switch.
outbound port Any port on a switch through which traffic leaves the switch.
outbound port queue For any port, a buffer that holds outbound traffic until it can leave the switch through that port. There is a "high priority" queue and a "normal priority" queue for each port in the switch . Traffic in a port's high priority queue leaves the switch before any traffic in the port's normal priority queue.
precedence bits The upper three bits in the Type of Service (ToS) field of an IP packet.
upstream device A device linked directly or indirectly to an inbound switch port. That is, the switch receives traffic from upstream devices.
802.1p priority A traffic priority setting carried only in packets in 802.1Q tagged VLANs. This setting can be from 0 to 7.
802.1Q tagged VLAN A virtual LAN (VLAN) that complies with the 802.1Q standard and is configured as "tagged". (For more on VLANs, see the Management and Configuration Guide you received with your HP ProCurve switch.)

Basic Operation

Configuring a CoS policy in the switch affects switch internal traffic priorities at the outbound port, and (if 802.1Q VLANs are configured in your network) the priority settings in traffic leaving the switch. This enables control over traffic movement within the switch and control over the priority settings in packets going to downstream devices and applications that can use those settings.

Policy Option for Controlling Traffic Priorities at the Outbound Port

Each port in the switch has two outbound traffic queues; ‘‘normal’’ priority and ‘‘high’’ priority. (High-priority packets leave the switch port first. Normal-priority packets leave the switch port after the port’s high-priority queue is emptied.) With no CoS control, all traffic (with two exceptions) goes through the ‘‘normal’’ outbound port queues. However, with a CoS policy operating in your network, you can determine the outbound priority queue to which a packet is sent.

Policy Options for Extending Traffic Priority Control to Outbound Packets Traveling to Downstream Devices

If an outbound packet is in an 802.1Q tagged VLAN environment (that is, if the packet is assigned to a tagged VLAN on the outbound port), then the packet carries an 802.1p priority setting that was configured in the switch. This priority setting can range from 0 to 7, and can be used by downstream devices having up to eight queues. Thus, while packets within a switch move only at high or normal priority, they still can carry the 802.1p priority that can be used by downstream devices having more than two priority levels. Also, if the packet enters the switch with an 802.1p priority setting, CoS can override this setting if configured to do so.

Priority Settings for Outbound Packets

You can configure a CoS priority of 0 through 7 for an outbound packet. When the packet is then sent to a port, the CoS priority determines which outbound queue the packet uses.

CoS Setting
 
Outbound Port
Queue
Operation
 
0 - 3 normal priority Packets in this queue leave the port after the high-priority queue is emptied.
4 - 7 high priority Packets in this queue leave the port first.

If a packet is not in an 802.1Q tagged VLAN environment, the above settings control only to which outbound queue the packet goes, and no 802.1p priority is added to the packet. However, if the packet is in an 802.1Q tagged VLAN environment, then the above setting is also added to the packet as an 802.1p priority that can be used by downstream devices and applications, as shown in the Priority Mapping Table.

Criteria for Prioritizing Outbound Packets

You can configure CoS prioritization on the basis of five criteria ranked as follows:

  1. Device Priority (destination or source IP address)
  2. IP Type of Service (ToS) field
  3. Protocol Priority (IP, IPX, ARP, DEC LAT, AppleTalk, SNA, and NetBeui)
  4. VLAN Priority
  5. Incoming 802.1p Priority (present in tagged VLAN environments)

If more than one criteria is present in a packet, the highest ranked of the criteria is used to prioritize the packet; all lower-ranked criteria are ignored. For example, if CoS assigns:

Since Protocol Priority (third in precedence, as shown above) has precedence over VLAN priority (fourth in precedence, as shown above), IP packets on the ‘‘red’’ VLAN will be set to normal priority. The "Priority Criteria and Precedence" table provides a more detailed description of how this operates.

Guidelines for Configuring CoS Priorities

You can use CoS regardless of whether the switch is in a tagged VLAN environment. As described earlier (under ‘‘Basic Operation’’):

To configure CoS, use this procedure:

  1. Determine the CoS policy you want to implement. This includes analyzing the types of traffic flowing through your network and identifying one or more traffic types to prioritize. If two or more of your policies apply to the same packet, then the switch applies only the policy having the highest precedence. The order of precedence for CoS criteria is as follows:

    1. Device Priority - destination or source IP address (Note that destination has precedence over source.)


    2. IP Type of Service (ToS)


    3. Protocol Priority


    4. VLAN Priority (requires at least one tagged VLAN)


    5. Incoming 802.1p Priority (requires at least one tagged VLAN)

    For more on how CoS operates with the above traffic types, see the "Priority Criteria and Precedence" table)

  2. If you want 802.1p priority settings to be included in outbound packets, ensure that tagged VLANs are configured on the appropriate links. (For information on configuring tagged VLANs, see the Management and Configuration Guide you received with the switch--and also available in the Support area of HP's ProCurve website.)


  3. Determine the actual Class of Service configuration changes you will need to make to establish a CoS policy for the switch.


  4. Configure the desired CoS policy.


Note: If you use TopTools for Hubs & Switches to configure Quality of Service (QoS) policy in a network, it overrides any CoS settings configured through the console or the web browser interface in any individual HP ProCurve switch.

For More CoS Information

Steps for Configuring CoS Priority

CoS Options Priority Configuration Steps
Device Priority Click on the Device Priority button, then:

To add an IP address:
  1. Type the address in the IP Address field.
  2. Select the desired priority level from the Priority pull-down menu.
  3. Click on the Add button.
To change a configured priority for a device:
  1. Type the device’s IP address in the IP Address field.
  2. Highlight a replacement priority level in the Priority pull-down menu.
  3. Click on the Replace button.
To delete a device from the Device Priority list:
  1. Click on the device’s IP address in the IP Address Priority field.
  2. Click on the Delete button.
Type of Service Click on the Type of Service button. Then:
  1. Use the pull-down menu to select either IP Precedence or Differentiated Services.
  2. Click on the Apply Changes button.
  3. If you selected Differentiated Services, you will then need to go to the Device View screen (under the Configuration tab) and Telnet to the switch console interface to change the priority for a given IP ToS value. For more on Type of Service, refer to Software Update C.07.XX Release Notes for the HP ProCurve Switch 1600M, 2400M, 2424M, 4000M, and 8000M.
Protocol Click on the Protocol button. Then:
  1. Click on the Priority pull-down menu for the desired protocol and select a priority level.
  2. Click on the Apply Changes button.
VLAN Priority Click on the VLAN Priority button. Then:
    Note: This feature configures the priority on existing VLANs (including the default VLAN). To configure new VLANs, go to the Device View screen (under the Configuration tab) and Telnet to the switch console interface.
  • Click on (highlight) the VLAN for which you want to configure a priority.
  • In the Priority pull-down menu, select the priority level you want.
  • Click on Modify VLAN priority.

 


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