Allied Telesis AT-S97 - Manual (Page 83)

AT-S85 and AT-S97 Management Software User’s Guide
83
disabling the transmitter on the companion port. This notifies the network
device connected to the port of the loss of the link on the other channel
port. Without the MissingLink mode, a network device connected to a
channel port is unaware of a loss of a link on its companion port in the
channel, because its link to the media converter is otherwise unaffected. In
other words, the MissingLink mode does not allow a port in a channel to
form a link with its network device unless the companion port can also
establish a link with its device.
When the link is reestablished on a channel port, the MissingLink mode
automatically reactivates the transmitter on the companion port so that
both network devices can again forward traffic to each other through the
two ports of the media converter channel.
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
on a port 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
ports of a channel are connected to two Fast Ethernet switches, one local
and the other remote. Switch A, the local switch, is connected to the
twisted pair port of the channel, while Switch B, the remote device, is
connected to the fiber optic port. If the link to Switch A is lost on the twisted
pair cable, the media converter disables the transmitter on the fiber optic
port in the same channel to signal Switch B of the loss 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 continue to
assume it still has a valid link to the remote device on the other side of the
media converter channel since its link to the port on the channel is still
valid, though no remote traffic is received.
In the example, the initial loss occurred on the twisted pair port. But the
mode operates the same when the initial loss of the link is on the fiber
optic port of a channel. Here, the transmitter on the twisted pair port is
disabled to notify the node connected to that port of the loss of the link on
the fiber optic port.
The states of the ports in a channel running in this mode operate in
tandem. Either both ports have a link or neither port does. This is reflected
on the Link LEDs and in the SYSTEM SHOW INTERFACE command. If
both ports can form a link with their respective network device, then their
Link LEDs will both be on and their status in the SYSTEM SHOW
INTERFACE command will be Online. If one or both ports cannot establish
a link, then their Link LEDs will both be off and their status in the SYSTEM
SHOW INTERFACE command will be Offline.