3Com 4500G Series, Configuration Manual

Page: 81 / 1206

1-5
Initially, each STP-enabled device on the network assumes itself to be the root bridge, with the root
bridge ID being its own device ID. By exchanging configuration BPDUs, the devices compare their root
bridge IDs to elect the device with the smallest root bridge ID as the root bridge.
z
Selection of the root port and designated ports on a non-root device
Table 1-3 describes the process of selecting the root port and designated ports.
Table 1-3
Selection of the root port and designated ports
Step Description
1
A non-root-bridge device regards the port on which it received the optimum
configuration BPDU as the root port.
2
Based on the configuration BPDU and the path cost of the root port, the device
calculates a designated port configuration BPDU for each of the rest ports.
z
The root bridge ID is replaced with that of the configuration BPDU of the root port.
z
The root path cost is replaced with that of the configuration BPDU of the root port
plus the path cost of the root port.
z
The designated bridge ID is replaced with the ID of this device.
z
The designated port ID is replaced with the ID of this port.
3
The device compares the calculated configuration BPDU with the configuration BPDU
on the port of which the port role is to be defined, and acts depending on the comparison
result:
z
If the calculated configuration BPDU is superior, the device considers this port as the
designated port, and replaces the configuration BPDU on the port with the calculated
configuration BPDU, which will be sent out periodically.
z
If the configuration BPDU on the port is superior, the device blocks this port without
updating its configuration BPDU. The blocked port can receive BPDUs but not send
BPDUs or forward data.
When the network topology is stable, only the root port and designated ports forward traffic, while other
ports are all in the blocked state – they receive BPDUs but do not forward BPDUs or user traffic.
A tree-shape topology forms upon successful election of the root bridge, the root port on each non-root
bridge and the designated ports.
The following is an example of how the STP algorithm works. As shown in Figure 1-2 , assume that the
priority of Device A is 0, the priority of Device B is 1, the priority of Device C is 2, and the path costs of
these links are 5, 10 and 4 respectively.