Panasonic PN28240i-ID Operation Manual page 264

Precautions in using loop detection function
– Disable loop detection at upstream port(s)
If a network is consisted of only Switching Hub equipped with loop
detection function, an upstream Switching Hub may detect on ahead and
block a loop occurred in a downstream Switching Hub. This may block all
communications to the downstream Switching Hub.
To minimize the communication failure by loop detection, disable the loop
detection function of the upstream Switching Hub so that only a port of the
Switching Hub causing loop will be blocked. You need to examine this type
of network configuration and Switching Hub settings.
Upstream
Switching Hub
Downstream
Switching
Hub 1
PC 1
[Conditions of occurrence]
(1) Communications between PC1 and PC2, and between PC1 and PC3 are OK.
(2) A loop occurs in downstream Switching Hub 2.
(3) The upstream Switching Hub detects the loop first, and shuts down the uplink port to
downstream Switching Hub 2. (Normally, only a port having loop connection is shut down.)
(4) The uplink port to downstream Switching Hub 2 is linked down, and communications to all
other Switching Hubs are blocked.
Fig. 2 Precautions in using loop detection function
(3)
(1)
264
(4)
Downstream
Switching
Hub 2
(2)
PC 2 PC 3