Limitations; Additional Notes - HP AdvanceStack 10Base-T Hub-12 Installation And Reference Manual

Limitations

The hubs at both ends of the redundant link must have an
HP Ethernet SNMP Module installed.
Each hub can monitor a single backup link (only one backup link
can be configured on each hub). But, the hub may be at the remote
end of one backup link and at the monitoring end of a backup link to
a different hub.
A given hub should be connected to the remote end of no more than
two backup links. If it is functioning as the remote hub in more than
two backup links, it may not be able to respond to the test packets
fast enough when there is a high level of data traffic on the network
segments.
This ability to respond may be improved by increasing the time
between test packet transmissions on the monitoring hubs. For all
the backup links in which the remote hub is involved, the time
configured on the monitoring hubs for those links should be
increased by one second for each additional backup link beyond two
links. Add one to this count if the remote hub is also functioning as a
primary (monitoring) hub in a different link.

Additional Notes

Any port on the hub can be used for either the primary link or the
backup link.
Any combination of media types can be used. For example, a thin
coax link from the ThinLAN port can act as a backup link to a
twisted-pair link using one of the hub's twisted-pair ports. A
transceiver installed in the hub's AUI/Xcvr slot, or externally
attached to an AUI Port Module in the AUI/Xcvr slot, can provide a
primary or backup link over any type medium (twisted-pair, coax, or
fiber-optic cable). (See "Sample Topologies" later in this appendix.)
The primary link and the backup link cabling should be run over
different paths (through different conduits, for example) to reduce
the possibility that damage will occur to both cables simultaneously.
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Backup Links
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