Allied Telesis AT-S63 - Manual (Page 194)

Chapter 16: Group Link Control
194 Section II: Advanced Operations
If connectivity is lost on both ports 17 and 20, the downstream ports 24
and 25 are disabled.
Figure 22. Group Link Control Example 5
In the previous examples the ports of the groups on the switch are
connected to different devices, making it possible for downstream devices
to know whether or not there are links to upstream devices. Another
approach is to create groups of ports that connect to the same network
node. This is useful in network topologies where there are redundant static
port trunks or LACP trunks that are controlled by the spanning tree
protocol. If a primary trunk loses bandwidth capacity because connectivity
is lost on one or more of their links and there is a redundant trunk held in
the blocking state by the spanning tree protocol, it may be advantageous
to shut down an impaired trunk so as to activate a redundant trunk, to
restore full bandwidth.
Switch 1
Network
Switch 3
Switch 2
Switch 4
Upstream ports 17, 20
Downstream ports 24, 25