Specifications

B.7 FDDI Connection Rules
FDDI Overview B–13
B.7.2 How Rings are formed
FDDI rings are formed by the completion of physical connections between pairs of
ports. The ports signal various parameters that are necessary for the successful com-
pletion of the connections.
When a port attempts to form a connection with another port, it indicates both its
own port type, and whether it wishes to form a connection with the port type that it
senses at the other end of the link.
B.7.3 Undesirable Connections
One type of connection, the M Port-to-M Port connection, is always rejected. Other
types of connections, such as A port to A port and B port to B port connections are
undesirable, but can be formed if the connection rules of one of the stations allows
the connection.
B.7.4 Example
For example, if a Digital FDDI product attempts to form an A port to A port connec-
tion with another Digital FDDI product the connection is rejected.
The connection is rejected because Digital products are designed to avoid connection
to similar ports (both ports signal that they do not wish to connect to a remote A
port).
If the Digital FDDI product attempts to form an A port to A port connection to a non-
Digital station, and the non-Digital station’s A port signals that it wants to accept the
connection to the Digital A port, the Digital station honors the request (per the ANSI
standard’s rules for connection of ports).
The result of this type of connection is graphically shown in the following subsec-
tion.
B.7.5 Result
The resulting configuration (see Figure B–7) is that, although the stations are con-
nected, one station’s MAC address is in the primary ring, and the other station’s
MAC address is in the secondary ring.
Because the rings are isolated when in the Through state (refer to “Station States” on
p. B-11), the stations are unable to communicate, even though their ports have
formed a valid physical connection.