AT-9000 Switch Command Line User’s Guide
Section II: Basic Operations 175
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 682 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 40. 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.
To Next Page
Loading...
