Allied Telesis AT-S99 - Manual (Page 43)

AT-S73 and AT-S99 Management Software User’s Guide
43
MissingLink
Mode
The two ports of a media converter line card operating in this mode share
their “link” status with each other so that both ports on a line card and,
consequently, the network devices connected to the ports, are always
aware of a change to the status of the link of one of the ports on the card.
When a line card in the MissingLink mode detects the loss of a link on one
of its ports, it replicates the loss on the other port by disabling the port’s
transmitter. This notifies the network device connected to the port of the
loss of the link on the other port on the line card. Without the MissingLink
mode, a network device connected to a port on a card would be unaware
of a loss of a link on the companion port, because its link to the media
converter line card would be unaffected. To explain it another way, the
MissingLink mode will not allow a port to form a link with its network device
unless the companion port on the same line card can also establish a link
with its device.
When the link is reestablished on a port, the MissingLink mode
automatically reactivates the transmitter on the companion port so that the
two network devices can again forward traffic to each other through the
two ports of the media converter line card.
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 the
two ports on a media converter line card are connected to two Fast
Ethernet switches, one local and the other remote. 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 is lost, the line card
disables the transmitter on port B 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
be unaware of the problem because it would still have a valid 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