3Com 3C16630A User Manual page 57

Repeater Management 3-29
Resilient Links are only available on twisted pair and fiber optic media,
because the link test pulse or the idle signal is used to decide whether
the link is broken and the standby link should be activated. There is no
such link signal over coaxial media.
If you have not come across resilience in the networking field before,
you will find the following explanation helps.
When a link is broken, all communication between devices on each
side of the link is lost. It could be very inconvenient for a manager
physically to reinstate the network immediately. Important traffic might
be lost. If a spare link could pick up where the broken link left off, the
network would appear to function normally to the outside observer. At
worst, a few frames would be corrupted or lost.
This is the concept of resilience as applied to ports. One port is on
stand-by waiting to take over if the main port fails.
You may set up 16 resilient pairs of ports on one chassis. A pair may be
on the same module or on different modules. Each port in the pair can
be attached to different media. Each resilient pair consists of a main
port and a stand-by port. When the pair is operating normally, the
main port carries data to and from the segment attached to the port.
However, if the Module detects a loss of link (link test pulse for twisted
pair medium) or loss of light (idle signal for fiber optic medium), this
main port is disabled and the stand-by port takes over.
To keep you informed of what is happening on the network, an event
or trap will be sent to the Network Manager informing you that a main
port has been disabled and a stand-by port has taken over. If you then
rectify the fault on the main port, or the fault clears, the operation will
switch back automatically to the main port.
It is important to ensure that the unit with the standby port has the
hardware switch (see Figure 2-1) in the correct position, that is, in the
'disabled on boot' position. If you are unsure of how to do this, consult
the guide that came with the Hub 10 unit. Setting the switch correctly
will avoid creating a loop when the power is first applied. The Module
will re-configure the ports after power-up.
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