Allied Telesis AT-S63 - Manual (Page 111)

AT-S63 Management Software Menus Interface User’s Guide
Section I: Basic Features 111
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
be 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 28. 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 28. 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.
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%