Allied Telesis AT-S102 - Manual (Page 50)

Chapter 2: AT-CM and AT-CV Media Converter Line Cards
50
The value to this type of fault notification is that some network devices,
such as managed Fast Ethernet switches, can respond to the loss of a link
by performing a specific action. For example, the network device might
send a trap to a network management station, and so alert the network
administrator of the problem. Or, if the device is running a spanning tree
protocol, it might seek a redundant path to a disconnected node.
Here is an example of how the MissingLink mode works. Assume that two
Fast Ethernet switches, one local and one remote, are connected to the
two ports on a media converter line card. Switch A, the remote switch, is
connected to port A on the line card, while Switch B, the local device, is
connected to port B. If the link to Switch A cannot be established or is lost,
the line card disables the transmitter on port B to signal Switch B of the
lack of the link to Switch A. This notifies Switch B of the problem so it too,
along with Switch A, 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 B would be unaware of the problem because it would still have a
link to the media converter line card.
In the example the initial loss occurred on port A. But the operating mode
operates the same when the initial loss of the link is on port B. Here, the
transmitter on port A is disabled to notify the node connected to that port of
the loss of the link on port B.
The states of the ports on a line card 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 and in the management software. If both ports
can form links with their network devices, their Link LEDs will both be on
and their status in the management software will be Online. If one or both
ports cannot establish a link, then the Link LEDs will be off and their status
in the management software will be Offline.
This operating mode is useful when the network devices connected to the
ports of a line card 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 a line card cannot react to a lost link. In the latter scenario, the
Link Test mode would probably be a better operating mode for a line card
during normal network operations.
Furthermore, Allied Telesis does not recommend using the MissingLink
mode when troubleshooting a network problem that may be rooted in a
link problem. 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 and the Smart
MissingLink modes are more useful when troubleshooting a link problem.