08-ACL and QoS Configuration Guide

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03-Data buffer configuration
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Configuring data buffers

About data buffers

Data buffer types

Data buffers temporarily store packets to avoid packet loss.

The following data buffers are available:

·     Ingress buffer—Stores incoming packets when the CPU is busy.

·     Egress buffer—Stores outgoing packets when network congestion occurs.

·     Headroom buffer—Stores packets when the ingress buffer or egress buffer is used up.

Figure 1 shows the structure of ingress and egress buffers.

Figure 1 Data buffer structure

Cell resources

A buffer uses cell resources to store packets based on packet sizes. A cell resource provides 208 bytes. The buffer allocates one cell resource to a 128-byte packet and two cell resources to a 300-byte packet.

Fixed area and shared area

The cell resources have a fixed area and a shared area.

·     Fixed area—Partitioned into queues, each of which is equally divided by all the interfaces on a card, as shown in Figure 2. When congestion occurs or the CPU is busy, the following rules apply:

a.     An interface first uses the relevant queues of the fixed area to store packets.

b.     When a queue is full, the interface uses the corresponding queue of the shared area.

c.     When the queue in the shared area is also full, the interface discards subsequent packets.

The system allocates the fixed area among queues as specified by the user. Even if a queue is not full, other queues cannot preempt its space. Similarly, the share of a queue for an interface cannot be preempted by other interfaces even if it is not full.

·     Shared area—Partitioned into queues, each of which is not equally divided by the interfaces, as shown in Figure 2. The system determines the actual shared-area space for each queue according to user configuration and the number of packets actually received and sent. If a queue is not full, other queues can preempt its space.

The system puts packets received or sent on all interfaces into a queue in the order they arrive. When the queue is full, subsequent packets are dropped.

Figure 2 Fixed area and shared area

Restrictions and guidelines: Data buffer configuration

You can configure data buffers either manually or automatically by enabling the Burst feature. If you have configured data buffers in one way, delete the configuration before using the other way. Otherwise, the new configuration does not take effect.

Inappropriate data buffer changes can cause system problems. Before manually changing data buffer settings, make sure you understand its impact on your device. As a best practice, use the burst-mode enable command if the system requires large buffer spaces.

Data buffer tasks at a glance

To configure the data buffer, perform the following tasks:

·     Enabling the Burst feature

·     Configuring data buffers manually

·     (Optional.) Configuring data buffer monitoring

·     (Optional.) Configuring data buffer alarms

¡     Configuring alarm thresholds for the ingress or egress buffer

¡     Configuring alarm thresholds for the Headroom buffer

¡     Configuring packet-drop alarms

Enabling the Burst feature

About the Burst feature

The Burst feature enables the device to automatically allocate cell and packet resources. It is well suited to the following scenarios:

·     Broadcast or multicast traffic is intensive, resulting in bursts of traffic.

·     Traffic comes in and goes out in one of the following ways:

¡     Enters a device from a high-speed interface and goes out of a low-speed interface.

¡     Enters from multiple same-rate interfaces at the same time and goes out of an interface with the same rate.

The default data buffer settings are changed after the Burst feature is enabled. You can display the data buffer settings by using the display buffer command.

Procedure

1.     Enter system view.

system-view

2.     Enable the Burst feature.

In standalone mode:

burst-mode enable

By default, the Burst feature is disabled.

Configuring data buffers manually

About manual data buffer configuration

Each type of resources of a buffer, packet or cell, has a fixed size. After you set the shared-area size for a type of resources, the rest is automatically assigned to the fixed area.

By default, all queues have an equal share of the shared area and the fixed area. You can change the maximum shared-area space and the fixed-area for a queue. The unconfigured queues use the default settings.

Procedure

1.     Enter system view.

system-view

2.     Configure buffer assignment rules. Choose the options to configure as needed:

¡     Set the total shared-area ratio.

In standalone mode:

buffer egress [ slot slot-number ] cell total-shared ratio ratio

In IRF mode:

buffer egress [ chassis chassis-number slot slot-number ] cell total-shared ratio ratio

The default setting is 100%.

¡     Set the maximum shared-area ratio for a queue.

In standalone mode:

buffer egress [ slot slot-number ] cell queue queue-id shared ratio ratio

In IRF mode:

buffer egress [ chassis chassis-number slot slot-number ] cell queue queue-id shared ratio ratio

The default setting is 20%.

The actual maximum shared-area space for each queue is determined based on your configuration and the number of packets to be received and sent.

¡     Set the fixed-area ratio for a queue.

In standalone mode:

buffer egress [ slot slot-number ] cell queue queue-id guaranteed ratio ratio

In IRF mode:

buffer  egress [ chassis chassis-number slot slot-number ] cell queue queue-id guaranteed ratio ratio

The sum of fixed-area ratios configured for all queues cannot exceed the total fixed-area ratio. Otherwise, the configuration fails.

3.     Apply buffer assignment rules.

buffer apply

You cannot directly modify the applied configuration. To modify the configuration, you must cancel the application, reconfigure data buffers, and reapply the configuration.

Configuring data buffer monitoring

About data buffer monitoring

The data buffer on a switch is shared by all interfaces for buffering packets during periods of congestion.

This feature allows you to identify the interfaces that use an excessive amount of data buffer space. Then, you can diagnose those interfaces for anomalies.

You can set a per-interface buffer usage threshold. The buffer usage threshold for a queue is the same as the per-interface threshold value. The switch automatically records buffer usage for each interface. When a queue on an interface uses more buffer space than the set threshold, the system counts one threshold violation for the queue.

Procedure

1.     Enter system view.

system-view

2.     Set a per-interface buffer usage threshold.

In standalone mode:

buffer usage threshold slot slot-number ratio ratio

In IRF mode:

buffer usage threshold chassis chassis-number slot slot-number ratio ratio

The default setting is 70%.

Configuring data buffer alarms

About data buffer alarms

This feature works with a network management system (such as IMC). Data buffer alarms include threshold-crossing alarms and packet drop alarms. The device reports these alarms to the network management system for displaying the data buffer usage.

Configuring alarm thresholds for the ingress or egress buffer

1.     Enter system view.

system-view

2.     Configure the alarm thresholds. Choose the options to configure as needed:

¡     Configure the global alarm threshold for a queue.

In standalone mode:

buffer { egress | ingress } usage threshold slot slot-number queue queue-id ratio ratio

In IRF mode:

buffer { egress | ingress } usage threshold chassis chassis-number slot slot-number queue queue-id ratio ratio

The default setting is 100%.

¡     Execute the following commands in sequence to configure the alarm threshold for a queue on an interface:

interface interface-type interface-number

buffer { egress | ingress } usage threshold queue queue-id ratio ratio

The default setting is 100%.

3.     Enable threshold-crossing alarms.

buffer threshold alarm { egress | ingress } enable

By default, threshold-crossing alarms are disabled.

4.     (Optional.) Set the interval for sending threshold-crossing alarms.

buffer threshold alarm { egress | ingress } interval interval

By default, the global alarm threshold is used.

The alarm threshold and the threshold sending interval take effect only when threshold-crossing alarms are enabled.

Configuring alarm thresholds for the Headroom buffer

1.     Enter system view.

system-view

2.     Configure the alarm thresholds. Choose the options to configure as needed:

¡     Configure the global per-queue alarm threshold.

In standalone mode:

buffer usage threshold headroom slot slot-number ratio ratio

In IRF mode:

buffer usage threshold headroom chassis chassis-number slot slot-number ratio ratio

The default setting is 100%.

¡     Execute the following commands in sequence to configure the alarm threshold for a queue on an interface:

interface interface-type interface-number

buffer usage threshold headroom queue queue-id ratio ratio

By default, the global per-queue alarm threshold is used.

3.     Enable threshold-crossing alarms.

buffer threshold alarm headroom enable

By default, threshold-crossing alarms are disabled.

4.     (Optional.) Set the interval for sending threshold-crossing alarms.

buffer threshold alarm headroom interval interval

The default setting is 5 seconds.

The alarm threshold and the threshold sending interval take effect only when threshold-crossing alarms are enabled.

Configuring packet-drop alarms

About packet drop alarms

This feature allows the device to periodically send packet-drop information to the NMS.

Restrictions and guidelines

This feature does not take effect on the Headroom buffer.

Procedure

1.     Enter system view.

system-view

2.     Enable packet-drop alarms.

buffer packet-drop alarm enable

By default, packet-drop alarms are disabled.

3.     (Optional.) Set the interval for sending packet-drop alarms.

buffer packet-drop alarm interval interval

The default setting is 5 seconds.

This command takes effect only when packet-drop alarms are enabled.

Display and maintenance commands for data buffers

Execute display commands in any view.

 

Task

Command

Display buffer size settings.

In standalone mode:

display buffer [ slot slot-number ] [ queue [ queue-id ] ]

In IRF mode:

display buffer [ chassis chassis-number slot slot-number ] [ queue [ queue-id ] ]

Display data buffer usage.

In standalone mode:

display buffer usage [ slot slot-number ]

In IRF mode:

display buffer usage [ chassis chassis-number slot slot-number ]

Display buffer usage statistics for interfaces.

display buffer usage interface [ interface-type [ interface-number ] ] [ verbose ]

 

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