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Configuring data buffers
An interface stores outgoing packets in the egress buffer when congestion occurs.
An egress 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.
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 on a
device, as shown in Figure 31. When
congestion occurs, 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 space for the queue in 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 31.
The system determines the actual shared-area ratio for each queue
according to user configuration and the number of packets actually sent. If a queue is not full,
other queues can preempt its space.
The system puts packets received on all interfaces into a queue in the order they arrive. When
the queue is full, subsequent packets are dropped.
Figure 31 Fixed area and shared area
Configuration task list
You can configure data buffers either automatically by enabling the Burst feature or manually.
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.
To configure the data buffer, perform the following tasks:
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