Allied Telesis AT-S63 - Manual (Page 189)

AT-S63 Management Software Features Guide
Section II: Advanced Operations 189
Overview
Group link control is designed to improve the effectiveness of the
redundant systems in a network. It enables the switch to alert network
devices about problems they might not otherwise detect or respond to, so
that they can implement their redundant systems, automatically.
The feature works by duplicating the link states of ports on other ports. If a
port does not have a link or loses a link, the switch duplicates the link state
on one or more other ports by disabling them.
To use the feature, you create groups of ports. The ports in a group are
referred to as upstream and downstream ports. In networking parlance,
the term “upstream” points towards a network core and “downstream”
points towards the edge of a network. So an upstream port would be
connected to a device at or towards the core of a network while a
downstream port would be connected to a device at or leading to the edge
of a network.
These definitions may or may not apply to the ports in the groups that you
create with group link control. It all depends on how you use the feature. In
some cases, the upstream port of a group will indeed be connected to a
device that leads to a network core while the downstream port is
connected to a different device at or towards the edge of a network. But in
other cases, this might not be true because the ports are connected to the
same device.
Instead, it might be better to think of the upstream port of a group as the
control port because it determines the possible link states of the
downstream port. The switch allows the downstream port in a group to
establish a link to its network device only if the upstream port already has
a link to a network node. If the upstream port does not have a link or loses
its link, the switch disables the downstream port to prevent it from
establishing a link. This notifies the device connected to the downstream
port that there is no connectivity on the upstream port.
There are two basic approaches to using this feature. One approach is to
create groups of ports that lead to different devices on the switch. This
approach is useful with network servers. The second approach is to group
ports that go to the same device. This is useful with static port trunks and
LACP trunks in a spanning tree topology.
It should be noted that group link control does not control the switching of
packets within the switch. It is just about the link states of the ports and
about transferring the states to other ports.
This feature is illustrated in the following figures.