HP AdvanceStack 10Base-T Hub-12 Installation And Reference Manual page 110

Backup Links
Description
N o t e
The Backup Link function is available only for HP AdvanceStack
10Base-T Hubs in which an HP Ethernet SNMP Module (HP J2603A or
B) is installed.
In some network configurations a critical link exists, for example
between two workgroups that regularly share or exchange data over
the network. To maintain the integrity of such a critical link, the
HP AdvanceStack 10Base-T Hubs offer a backup link feature. A backup
link is a separate cable run between two hubs that is automatically
enabled if the connection designated as the primary link fails.
SNMP
hub A
"monitoring
hub"
The hub on which the redundant link is configured (hub A in the
illustration—the "Monitoring Hub"), is responsible for monitoring the
link. It sends IEEE 802.2 Test packets to the hub at the other end of the
link (hub B in the illustration—the "Remote Hub") and looks for
response packets from that hub. If the response packets fail to come
back, the primary link is considered as having failed and the backup
link, which had not been carrying any traffic, is enabled automatically. If
the primary link does fail, it is automatically disabled until it can be
repaired and re-enabled.
When the primary link is repaired, you must re-enable the primary port.
It is not re-enabled automatically. When the primary port is enabled, the
backup port is automatically disabled and returned to backup mode. See
"Reactivating the Primary Link" later in this appendix.
D-2
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802.2 test packets
802.2 response packets
"backup" link
(normally inactive)
SNMP
"primary" link
hub B
"remote
hub"