3Com 4007, Implementation Manual, Page: 178 / 684

178 C
HAPTER
9: B
RIDGE
-W
IDE AND
B
RIDGE
P
ORT
P
ARAMETERS
Determining the Root Bridge
The root ID portion of the CBPDU determines which bridge actually
becomes the root bridge. In Figure 5, notice how each bridge initially
assumes that it is the root bridge and transmits a CBPDU that contains its
own bridge ID as both the
root ID and the transmitting bridge ID as well
as a
zero cost
. In Figure 6, because Bridge B has the lowest root ID of all
the bridges, it becomes the root and all other bridges change their root ID
to Bridge B’s ID (10).
Determining the Root Ports
Next, each bridge (except for the root bridge) must select a root port. To
select a root port, each bridge determines the most cost-effective path
for frames to travel from each of its ports to the root bridge. The cost
depends on:
■
The port path cost
■
The root path cost of the designated bridge for the LAN to which this
port is attached
If the bridge has more than one port attachment, the port with the
lowest cost becomes the root port, and the other ports become either
designated or backup ports. If bridges have redundant links to the same
LAN, then the port with the lowest port identifier becomes the root port.
In Figure 6, Bridge F has two links to LAN 3 (through port 1 and port 2).
Because the lowest port identifier for Bridge F is port 1, it becomes the
root port, and port 2 becomes a backup port to LAN 3.
Determining the Designated Bridge and Designated Ports
For a LAN attached to a single bridge, that bridge is the LAN’s designated
bridge. For a LAN that is attached to more than one bridge, a designated
bridge must be selected from among the attached bridges.
The root bridge is automatically the designated bridge for all of its directly
attached LANs.
For example, Bridge B, the root bridge in Figure 6, is also the designated
bridge for LANs 1, 2, and 5.