Configuring Congestion Avoidance (Tail Drop Profile Mode)

Context

The switch uses tail drop to prevent congestion. When the length of a queue reaches the maximum value, all the packets last added to the queue (at the tail of the queue) are discarded. You can set the maximum number of bytes or packets to be cached in a queue to prevent traffic loss.

Congestion avoidance can be configured in tail drop profile mode only on the S2720-EI, S5720I-SI, S5720-LI, S5720S-LI, S5720S-SI, S5720-SI, S5730S-EI, S5730-SI, S5735-L, S5735S-L, S5735S-L-M, S5735-S, S5735-S-I, S5735S-S, S6720-LI, S6720S-LI, S6720S-SI, and S6720-SI.

Before setting the maximum number of bytes or packets to be cached in a queue, run the shutdown command to shut down the interface. After the maximum number of bytes or packets to be cached in a queue is set, run the undo shutdown command to enable the interface. These operations may cause network interruption in a short period of time.

Procedure

Run system-view
The system view is displayed.
Run qos tail-drop-profile profile-name
A tail drop profile is created and the tail drop profile view is displayed.

Only one tail drop profile can be created on the S5735-L, S5735S-L, S5735S-L-M, S5735-S, S5735-S-I, and S5735S-S.
Run the following commands as required.
S2720-EI, S5720I-SI, S5720-LI, S5720S-LI, S5720S-SI, S5720-SI, S5730S-EI, S5730-SI, S6720-LI, S6720S-LI, S6720S-SI, and S6720-SI:
- Run qos queue queue-index max-buffer cell-number [ green max-buffer cell-number ]
  
  The maximum number of bytes to be cached in a queue is set.
- Run qos queue queue-index green max-buffer cell-number
  
  The maximum number of bytes to be cached in a queue is set.
- Run qos queue queue-index max-length packet-number [ green max-length packet-number ]
  
  The maximum number of packets to be cached in a queue is set.
- Run qos queue queue-index green max-length packet-number
  
  The maximum number of packets to be cached in a queue is set.
If the maximum number of bytes or packets is reached, the device considers that congestion occurs and will discard subsequent packets.

S5735-L, S5735S-L, S5735S-L-M, S5735-S, S5735-S-I, and S5735S-S:
- Run qos queue queue-index low-limit low-limit-value high-limit high-limit-value
  The upper and lower limits for buffering packets in a queue are set.
  
  By default, the upper and lower limits for buffering packets in a queue are 78 and 68 buffer units, respectively. Each buffer unit is 360 bytes.
  When the size of packets in the queue buffer is always lower than or equal to the lower limit (low-limit low-limit-pages), the switch does not discard any packet. When the size of packets in the buffer increases and is greater than the lower limit:
  - If the size of packets in the queue buffer is always less than or equal to the upper limit (high-limit high-limit-pages), the switch does not discard any packet.
  - If the size of packets in the buffer increases gradually or even exceeds the upper limit, the switch discards the packets in the buffer until the size of packets in the queue does not exceed the lower limit.
Run quit
Return to the system view.
Run interface interface-type interface-number
The interface view is displayed.
Run shutdown
The interface is shut down.
Run qos tail-drop-profile profile-name
The tail drop profile is applied to the interface.
Run undo shutdown
The interface is restarted.

Verifying the Configuration

Run the display qos configuration interface interface-type interface-number command to check all the QoS configurations on the interface.
Run the display qos queue statistics interface interface-type interface-number [ queue queue-index ] command to check the queue-based traffic statistics on the interface.