Contents

Configuring data buffers· 1

About data buffers· 1

Data buffer types· 1

Cell resources and packet resources· 1

Fixed area and shared area· 1

Restrictions and guidelines: Data buffer configuration· 2

Enabling the Burst feature· 2

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.

Figure 1 shows the structure of ingress and egress buffers.

Figure 1 Data buffer structure

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Cell resources and packet resources

A buffer uses the following types of resources:

·     Cell resources—Store packets. The buffer uses cell resources based on packet sizes. Suppose 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.

·     Packet resources—Store packet pointers. A packet pointer indicates where the packet is located in cell resources. The buffer uses one packet resource for each incoming or outgoing packet. Packet resources are not supported in the current software version.

Fixed area and shared area

Each type of resources has a fixed area and a shared area.

·     Fixed area—Partitioned into queues, each of which is equally divided by all the interfaces, 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

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

Enabling the Burst feature

The Burst feature enables the device to automatically allocate buffer 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.

A switch enabled with the Burst feature supports only WRR queuing. To use other queuing methods, first disable the Burst feature.

To enable the Burst feature: