Allied Telesis AT-S62 - Manual (Page 101)

AT-S62 Menus Interface User’s Guide
Section I: Basic Operations 101
Note
Ports 49R and 50R on an AT-8550GB Series switch must be set to
Auto-Negotiation in order to operate at 1000Mbps. You cannot
manually configure these ports to 1000Mbps.
6 - Duplex
This selection is used to set the duplex mode of a port. The option
only appears when option 4 - Negotiation is set to Manual. The
possible settings are:
Full Full-duplex
Half Half-duplex.
7 - HOL Blocking Prevention Threshold
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.
The problem an oversubscribed port can create is that it can
prevent other ports from forwarding packets to each other. This is
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 will
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 23. 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. The result is that 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.