Allied Telesis AT-S63 - Manual (Page 65)

AT-S63 Management Software Menus User’s Guide
Section I: Basic Operations 65
Configuring Head of Line Blocking
Head of line (HOL) blocking is a problem that occurs when a port on a
switch becomes oversubscribed. An oversubscribed port 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 the head of an ingress queue consists of a
packet destined for an oversubscribed port, the ingress queue is not able
to forward any of its other packets to the egress queues of other ports.
A simplified version of the problem is illustrated in Figure 17. It shows four
ports on a switch. Port D is receiving packets from two ports—50% of the
ingress traffic on port A and 100% of the ingress traffic on port B. Not only
is port A unable to forward packets to port D because the latter’s egress
queues are filled with packets from port B, but it is also unable to forward
traffic to port C because its ingress queue has frames destined to port D
that it is unable to forward.
Figure 17. Head of Line Blocking
The HOL Limit parameter can help prevent this problem from occurring.
This parameter sets a threshold on the utilization of a port’s egress queue.
When the threshold for a port is exceeded, the switch signals other ports to
discard packets to the oversubscribed port.
For example, referring to the figure above, when the utilization of the
storage capacity of port D exceeds the threshold, the switch signals the
Port A
C C C C D D D D
Ingress Queue
Port B
D D D D D D D D
Ingress Queue
Port C
Egress Queue
Port D
D D D D D D D D
Engress Queue
100%
50%
50%