3Com 4007 Implementation Manual page 172

172 C 9: B
HAPTER RIDGE
-W B P P
IDE AND RIDGE ORT ARAMETERS
Bridges elect as the designated bridge on each LAN the bridge with
the least cost path to the root bridge. The designated bridge forwards
frames between that LAN and the path to the root bridge. For this
reason, the root bridge is always the designated bridge for its
attached LANs. The port through which the designated bridge is
attached to the LAN is elected the designated port.
Bridges choose a root port that gives the best path from themselves to
the root bridge.
Bridges select ports to include in the STP topology. The ports that are
selected include the root port plus any designated ports. Data traffic is
forwarded to and from ports that have been selected in the STP
topology.
Figure 4 shows a bridged network with its STP elements.
Figure 4 STP Root and Designated Bridges and Ports
Designated
bridge for
LAN 3
Designated
bridge for
LANs 1, 2, and 4
(D)
Bridge
(R)
LAN 1
(D)
(D)
Root bridge
(D)
LAN 2
LAN 3
(R)
Bridge
(B)
LAN 4
(R) = Root port
(D) = Designated port
(B) = Backup port