Cabletron Systems SPECTRUM, User Manual

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Configuring the SMT Connection Policy 4-23
FDDI Management
Each interface controls only its own connection policy; however, when two
interfaces attempt to connect, their combined connection policies dictate the
connections that will be allowed, with the most lenient policy prevailing — in
other words, all connections (except for the illegal M—>M connection) are
allowed unless forbidden by both connecting nodes. For example, if you disallow
the A—>M connection on one node, but attempt to make that connection with
another node which does not forbid it, the connection will be allowed.
Special Ring Configurations
You can use the SMT Connection Policy window to allow or prevent the following
ring configurations:
Dual Homing
Dual homing is a method of configuring concentrators with a redundant topology
that provides a backup data path to protect critical devices from losing contact
with the main ring; dual homing also achieves a kind of separation from the main
ring that makes it easy to bring a critical device down for maintenance without
causing widespread ring failure.
To achieve a dual homing configuration, connect the A port of your critical device
to an M port on one dual-attached concentrator (DAC), and connect the B port of
the same device to an M port on another DAC. SMT will automatically make the
B—>M connection active and place the A—>M connection in stand-by; the
A—>M connection will only become active if the B—>M connection should fail.
(Once the B—>M connection is restored, it is automatically re-activated, and the
A—>M connection goes back into standby mode.) Dual homing will not be
permitted if either the A—>M or B—>M connections have been disallowed for all
involved nodes.
Twisted Ring
When an FDDI ring is in a twisted configuration, at least one station is supporting
both
an A—>A connection and a B—>B connection; in this configuration, the
station with the A—>A and B—>B connections is actually residing on the
secondary FDDI ring, and is therefore isolated from the stations on the primary
ring. A wrap condition on a twisted ring will bring the isolated station back into
contact with any stations still connected to the primary ring. You can prevent a
twisted ring configuration by disallowing the A—>A and/or the B—>B
connections for all nodes.
Defining Your Connection Policy
To configure the connection policy for the selected interface:
1. To disallow any connection types, click mouse button 1 on the appropriate
selection box or boxes; to allow connections which have been previously
disallowed (except for the illegal M—>M connection), click on the selection
box again.