Configuring data buffers 1

Overview·· 1

Buffer resource· 1

Shared area and fixed area· 1

Configuring a data buffer 2

Setting the shared-area size· 2

Applying data buffer configuration· 3

Displaying and maintaining data buffers 3

Overview

Data buffers temporarily store packets to avoid packet loss.

An interface card has an ingress buffer and an egress buffer. An interface stores outgoing packets in the egress buffer when congestion occurs, and it stores incoming packets in the ingress buffer when the CPU is busy.

Buffer resource

As the smallest unit of a buffer, buffer resources are used to store packets. An interface uses buffer resources based on packet sizes. Suppose a buffer resource provides 512 bytes. The interface allocates one buffer resource to a 504-byte packet and two buffer resources to a 520-byte packet. The switch provides 512 bytes per buffer resource for the egress buffer and 1024 bytes per buffer resource for the ingress buffer.

Shared area and fixed area

Both an ingress buffer and an egress buffer have a shared area and a fixed area. The shared area is partitioned into queues, as shown in Figure 1.

·     The shared area is shared by the queues of all interfaces. When an interface or a queue receives traffic bursts, it can temporarily preempt buffer resources of the shared area. After sending out the traffic bursts, the interface releases the preempted resources for use by other interfaces or queues. You can manually set the number of buffer resources for the shared area, and the rest is automatically assigned to the fixed area.

·     The fixed area is allocated to interfaces and then to queues of each interface. First, the switch allocates the buffer resources of the fixed area equally among all interfaces on an interface card. Then, the switch allocates the buffer resources of each interface equally among all its queues. In this manner, all queues of all interfaces have an equal number of dedicated buffer resources.

Figure 1 Shared area and fixed area

When congestion occurs, the following rules apply:

1.     An interface first uses the buffer resources of the shared area to store packets.

2.     When no buffer resources are available in the shared area, the interface uses the buffer resources of the corresponding queue in the fixed area.

3.     When no buffer resources of the queue in the fixed area are available, the interface discards subsequent packets.

When an interface uses the egress buffer, the following rules apply:

·     Packets with drop precedence 0 can use buffer resources in both the shared area and fixed area.

·     Packets with drop precedence 1 or 2 can use only buffer resources in the fixed area.

Configuring a data buffer

Setting the shared-area size

The data buffer on an interface card has a fixed size. After you set the shared-area size for a data buffer, the rest is automatically assigned to the fixed area.

To set the shared-area size:

Applying data buffer configuration

Perform this task to apply the data buffer configuration.

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

To apply data buffer configuration:

Displaying and maintaining data buffers

Execute display commands in any view.