KYLAND Technology SICOM3024P, Руководство по работе в сети

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Advanced Configuration
93
6.13.3 BPDU
To prevent loops, all the bridges of a LAN calculate a spanning tree. The calculation process
involves transmitting BPDUs among devices to determine the network topology. The
following table shows the data structure of a BPDU.
Table 6 BPDU
…
Root
bridge ID
Root path
cost
Designated
bridge ID
Designated
port ID
Message
age
Max
age
Hello
time
Forward
delay
…
…
8 bytes 4 bytes 8 bytes 2 bytes 2 bytes 2 bytes 2 bytes 2 bytes
…
Root bridge ID: priority of the root bridge (2 bytes)+MAC address of the root bridge (6 bytes).
Root path cost: cost of the path to the root bridge.
Designated bridge ID: priority of the designated bridge (2 bytes)+MAC address of the
designated bridge (6 bytes).
Designated port ID: port pport number.
Message age: duration that a BPDU can be spread in a network.
Max age: maximum duration that a BPDU can be saved on a device. When Message age is
larger than Max age, the BPDU is discarded.
Hello time: interval for sending BPDUs.
Forward delay: status change delay (discarding--learning--forwarding).
6.13.4 Implementation
The process for all bridges calculating the spanning tree with BPDUs is as follows:
1. In the initial phase, each port of all devices generates the BPDU with itself as the root
bridge; both root bridge ID and designated bridge ID are the ID of the local device; the
root path cost is 0; the designated port is the local port.
2. Best BPDU selection: All devices send their own BPDUs and receive BPDUs from other
devices. Upon receiving a BPDU, each port compares the received BPDU with its own.

If the priority of its own BPDU is higher, then the port does not perform any operation.

If the priority of the received BPDU is higher, then the port replaces the local BPDU with