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Chapter 17 MSTP Configuration
17.1.1.2 Bridge Protocol Data Units (BPDU):
To span a stable tree-type topology, it should depend on the elements below:
The unique bridge ID of each bridge consists of the bridge priority and the MAC
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address.
The bridge to root path cost is short for the Root Path Cost.
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Each port ID consists of the port priority and port number.
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The information required to establish the optimal tree-type topology is obtained by the
switching BPDU (Bridge Protocol Data Units) among bridges. These frames take the
multicast address 01-80-C2-00-00-00 (hex) as the destination address.
Each BPDU is comprised of the following elements:
Root Bridge ID (the root bridge ID this bridge considers)
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Root Path cost (the Root Path cost of this bridge).
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Bridge ID (the bridge ID of this bridge).
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Message age (the live time of the packet)
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Port ID (the port ID that the port sends this packet).
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The time parameters of the Forward-Delay Time, the Hello Time and the Max-Age Time
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protocol.
Other flag bits, such as those represent to detect the change of the network topology
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and the status of this port.
When one port of the bridge receives the BPDU with higher priority (the smaller bridge ID
and less root path cost), this information will be stored at this port. At the same time, it will
update and promulgate this information for all ports. If the BPDU with lower priority is
received, the bridge will discard this information.
This mechanism makes the information with higher priority be spreaded in the whole
network, and the exchange of the BPDU will obtain the following results:
One bridge is taken as the Root Bridge in the network.
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Each bridge other than the root bridge will present a Root Port. Namely, it will provide
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the port to the Root Bridge with the shortest path.
Each bridge will calculate the shortest path to the Root Bridge.
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Each LAN will present the Designated Bridge, which lies in the shortest path between
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this LAN and the root bridge. The port for connecting the Designated Bridge and the
LAN is referred to as the Designated port.
The Root port and the Designated port enter the Forwarding status.
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Other ports that will not span the tress will be in the Discarding status.
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17-2
DGS-3610 Series Configuration Guide