3Com CoreBuilder 3500, Implementation Manual, Page: 128 / 592

128 C
HAPTER
7: B
RIDGE
-W
IDE AND
B
RIDGE
P
ORT
P
ARAMETERS
For example, Bridge B, the root bridge in Figure 23, is also the designated
bridge for LANs 1, 2, and 5.
A designated bridge must be determined for LANs 3, 4, and 6:
■
Because Bridges C, D, and F are all attached to LAN 3, one of them
must be the designated bridge for that LAN:
■
The algorithm first compares the root ID of these bridges, which is
the same for all.
■
The cost is then compared. Bridge C and Bridge D both have a cost
of 11. Bridge F, with a cost of 12, is eliminated as the designated
bridge.
■
The transmitting bridge ID is compared between Bridge C and
Bridge D. Because Bridge C’s ID (20) is smaller than Bridge D’s (29),
Bridge C becomes the designated bridge for LAN 3.
■
The designated bridge for LAN 6 is either Bridge D or Bridge E.
Because Bridge D’s transmitting bridge ID (29) is lower than Bridge E’s
(35), Bridge D becomes the designated bridge for that LAN.
■
The designated bridge for LAN 4 is Bridge F, the only bridge that is
attached to that LAN.
The port that attaches the designated bridge to the LAN determines the
designated port. If more than one port is attached to the same LAN, then
the port identifier determines the designated port.