Справочник Пользователя для Enterasys Networks E1 Series

At times during normal switch operation, a burst of traffic could temporarily oversubscribe an 
egress port. Oversubscribed means more traffic is destined to a port than it can transmit. The two 
general approaches to handle this situation are flow control and Head of Line (HOL) Blocking 
Prevention.

Exerting flow control causes the oversubscribed port to inform the port or ports transmitting to the 
congested port to temporarily stop sending frames so the egress port can “catch up”. This has the 
side effect of preventing the senders from sending any frames — not just frames to the congested 
destination port. Thus, flow control can negatively affect traffic to uncongested ports.

Head of Line Blocking Prevention uses a different approach. Head of Line blocking occurs when a 
switch can’t accept frames because frames already in the system can’t leave fast enough, causing 
congestion. When enabled, Head of Line Blocking Prevention drops congested frames unable to 
leave the switch, allowing it to always accept new frames. Instead of exerting flow control, HOL 
Blocking Prevention drops frames after a pre-defined number of frames are queued to the congested 
port. This prevents flow control from hampering other uncongested ports at the expense of dropping 
frames to the congested port.

To configure port flow control, buffer controls and Head of Line (HOL) Blocking Prevention 
thresholds. 

The commands needed to set port flow control and thresholds are listed below and described in the 
associated section as shown.

)

show port buffer threshold (

)

set port buffer threshold (

)

CAUTION: Port threshold configuration should be performed only by personnel who 
are knowledgeable about the effects of setting thresholds and its impact on network 
operation.