3Com 5500-SI Configuration Manual page 540

540 C 26: RSTP C
HAPTER ONFIGURATION
For a Switch, the designated bridge is a Switch in charge of forwarding BPDU to the
local Switch using a port called the designated port. For a LAN, the designated bridge
is a Switch that is in charge of forwarding BPDU to the network segment using a port
called the designated port. As illustrated in Figure 159, Switch A forwards data to
Switch B using the port AP1. So to Switch B, the designated bridge is Switch A and
the designated port is AP1. Also in Figure 159, Switch B and Switch C are connected
to the LAN and Switch B forwards BPDU to LAN. So the designated bridge of LAN is
Switch B and the designated port is BP2.
AP1, AP2, BP1, BP2, CP1 and CP2 respectively delegate the ports of Switch A, Switch
B and Switch C.
The Specific Calculation Process of STP Algorithm
The following example illustrates the calculation process of STP.
Figure 160 illustrates the network.
Figure 160 Switch Networking.
To facilitate the descriptions, only the first four parts of the configuration BPDU are
described in the example. They are root ID (expressed as Ethernet Switch priority),
path cost to the root, designated bridge ID (expressed as Ethernet Switch priority) and
the designated port ID (expressed as the port number). As illustrated in Figure 160,
the priorities of Switch A, B and C are 0, 1 and 2 and the path costs of their links are
5, 10 and 4 respectively.
1 Initial state
When initialized, each port of the Switches will generate the configuration BPDU
taking itself as the root with a root path cost as 0, designated bridge IDs as their own
Switch IDs and the designated ports as their ports.
Switch A:
■
Configuration BPDU of AP1: {0, 0, 0, AP1}
Configuration BPDU of AP2: {0, 0, 0, AP2}
Switch B:
■
Configuration BPDU of BP1: {1, 0, 1, BP1}
Configuration BPDU of BP2: {1, 0, 1, BP2}
Switch C:
■
Configuration BPDU of CP2: {2, 0, 2, CP2}
Configuration BPDU of CP1: {2, 0, 2, CP1}
Switch A
with priority 0
AP1
5
BP1
Switch B
with priority 1
BP2
AP2
10
CP1
4
CP2
Switch C
with priority 2