Allied Telesis AT-9000 Series - Manual (Page 207)

AT-9000 Switch Command Line User’s Guide
179
HOLBPLIMIT
Syntax
holbplimit holbplimit
Parameter
holbplimit
Specifies the threshold at which a port signals a head of line
blocking event. The threshold is specified in cells. A cell is 128
bytes. The range is 1 to 8,191 cells; the default is 7,168 cells.
Mode
Port Interface mode
Description
Use this command to specify a threshold for head of line blocking events
on the ports. Head of line (HOL) blocking is a problem that occurs when a
port on the switch becomes oversubscribed because it is receiving more
packets from other switch ports than it can transmit in a timely manner.
An oversubscribed port can prevent other ports from forwarding packets to
each other because ingress packets on a port are buffered in a First In,
First Out (FIFO) manner. If a port has, at the head of its ingress queue, a
packet destined for an oversubscribed port, it will not be able to forward
any of its other packets to the egress queues of the other ports.
A simplified version of the problem is illustrated in Figure 48 on page 180.
It shows four ports on the switch. Port D is receiving packets from two
ports— 50% of the egress traffic from port A and 100% of the egress traffic
from port B. Not only is port A unable to forward packets to port D because
port D’s ingress queues are filled with packets from port B, but port A is
also unable to forward traffic to port C because its egress queue has
frames destined to port D that it is unable to forward.