Allied Telesis AT-PC2002/POE - Manual (Page 22)

Chapter 1: Overview
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connected to port 1, the fiber optic port, on the media converter, while
Switch 2, the local device, is connected to port 2, the twisted pair port. If
the link to Switch 1 is lost, the line card disables the transmitter on port 2 to
signal Switch 2 of the loss of the link to Switch 1. This notifies Switch 2 of
the problem so it too, along with Switch 1, can take remedial action, such
as activating a redundant path if it is running a spanning tree protocol, or
sending an SNMP trap to a management workstation. Without the
MissingLink mode, Switch 2 would be unaware of the problem because it
would still have a valid link to the media converter.
In the example the initial loss occurred on port 1. But the operating mode
operates the same when the initial loss of the link is on port 2. Here, the
transmitter on port 1 is disabled to notify the node connected to that port of
the loss of the link on port 2.
The states of the ports on the media converter running in this mode
operate in tandem. Either both of the ports have a link or neither of the
ports. This is reflected on the Link LEDs. If both ports can form links with
their network devices, their Link LEDs will both be on. But if one or both
ports cannot establish a link, then the Link LEDs will be off.
This operating mode is mainly used when the network devices connected
to the ports of the media converter can react to a loss of a link on a port,
such as managed Fast Ethernet switches running SNMP or a spanning
tree protocol. Conversely, the MissingLink mode will be of little value if the
network devices of the media converter cannot react to a lost link. In the
latter scenario, the Link Test mode would probably be a better operating
mode for the media converter during normal network operations.
Furthermore, Allied Telesis does not recommend using the MissingLink
mode to troubleshoot a network problem that may have its roots with a link
problem. This is because the MissingLink mode will not allow you to use
the port’s Link LEDs or the management software to diagnose the
problem, since neither port will show a link. Rather, the Link Test mode or
the Smart MissingLink mode would be more useful when troubleshooting
a link problem.
Smart
MissingLink
Mode
The Smart MissingLink mode is nearly identical to the MissingLink mode.
It, too, enables the two ports on the media converter to share the link
status of their connections. This mode is different in that it does not
completely shut off the transmitter of a port when the other port on the unit
loses its link. Rather, this operating mode pulses the port’s transmitter and
flashes the port’s Link LED once a second to signal that the port can still
establish a link to its network device and that the loss of the link originated
on the other port on the media converter.
The advantage of this operating mode over the MissingLink mode is that
you can use the Link LEDs on the media converter to troubleshoot a link
failure with the ports. As an example, assume that the fiber optic port on a
media converter lost its link to its network device. The mode would