3Com 4007, Implementation Manual, Page: 173 / 684

STP Terms and Concepts 173
Contents of CBPDUs
Bridges use information in CBPDU to calculate a STP topology. The
content of a CBPDU includes:
■
Root ID — The identification number of the root bridge.
■
Cost — The cost of the least-cost path to the root from the
transmitting bridge. One of the determining factors in cost is the
speed of the bridge’s network interface; that is, the faster the speed,
the lower the cost.
■
Transmitting bridge ID — The identification of the bridge that
transmits the CBPDU, which includes the bridge address and the
bridge priority.
■
Port identifier — Includes the port priority as well as the number of
the port from which the transmitting bridge sent the CBPDU.
The port identifier is used in the STP calculation only if the root IDs,
transmitting bridge IDs, and costs (when compared) are equal. In
other words, the port identifier is a tiebreaker in which the lowest port
identifier takes priority. This identifier is used primarily for selecting the
preferred port when two ports of a bridge are attached to the same
LAN or when two routes are available from the bridge to the root
bridge.
Comparing CBPDUs
Here are three examples that show how the bridge determines the best
CBPDU. In every case, the root ID is the most important determining
factor. If the root ID fields are equal, then the cost is compared. The last
determining factor is the transmitting bridge ID. If the CBPDUs all have
the same root ID, cost, and transmitting bridge ID, then the port identifier
is used as a tiebreaker.
Example 1. Root ID is lower for Message 1. The bridge saves Message 1.
Message 1 Message 2
root ID cost transmitter root ID cost transmitter
12 15 35 31 12 32