Port Configuration Command Set
Setting Flow Control and Thesholds
Matrix E1 Series (1G58x-09 and 1H582-xx) Configuration Guide 4-27
4.3.6 Setting Flow Control and ThesholdsAbout Managing Oversubscribed Ports
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.
Purpose
To configure port flow control, buffer controls and Head of Line (HOL) Blocking Prevention
thresholds.
Commands
The commands needed to set port flow control and thresholds are listed below and described in the
associated section as shown.
•show port flowcontrol (Section 4.3.6.1)
•set port flowcontrol (Section 4.3.6.2)
•show port buffer threshold (Section 4.3.6.3)
•set port buffer threshold (Section 4.3.6.4)
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.