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DES-7200 Configuration Guide Chapter 10 MSTP Configuration
10-2
10.1.1.2 Bridge Protocol Data Units
(BPDU):
A stable tree-type topology depends on the following elements :
z The unique bridge ID of each bridge consists of the bridge priority and the
MAC address.
z The root path cost refers to the cost from a bridge to the root bridge.
z Each port ID consists of the port priority and port number.
By exchanging the Bridge Protocol Data Units (BPDU) frame destined to the
multicast address 01-80-C2-00-00-00 (in hex), bridges gets the information
necessary for building the optimal tree-type topology.
A BPDU is comprised of the following elements:
z Root Bridge ID (root bridge ID that a bridge considers)
z Root Path cost (Root Path cost of a bridge).
z Bridge ID (ID of a bridge).
z Message age (the live time of the message)
z Port ID (port ID sending the message).
z Forward-Delay Time, Hello Time and Max-Age: time parameters.
z Other flag bits, such as network topology change and port status.
Once a port of a bridge receives a BPDU message whose priority is higher than
its priority (or smaller bridge ID and smaller root path cost), the bridge will store
this message on the port while updating and propagating them to all other ports.
If the BPDU with lower priority is received, the bridge will discard this message.
This mechanism propagates a BPDU message of higher priority in the whole
network. As a result:
z A bridge is elected to be the root bridge in the network.
z Each bridge other than the root bridge has a root port that offers a shortest
path to the root bridge.
z Each bridge will calculate the shortest path to the root bridge.
z Each LAN has a designated bridge that lies in the shortest path between
this LAN and the root bridge. The port for connecting the designated bridge
and the LAN is referred to as the designated port.
z The root port and the designated port are in the forwarding status.
z Other ports beyond the spanning tree are in the discarding status.
10.1.1.3 Bridge ID
As specified in IEEE 802.1W standard, each bridge has an unique bridge ID
based on which the root bridge is elected in spanning tree algorithm. The bridge
ID consists of eight bytes, in which the last six bytes are the MAC address of the
bridge, and the first two bytes are shown in the table below. Of which, the first