Calculating The Stp Algorithm; Generating The Configuration Bpdu - 3Com 7700 Configuration Manual

206 C 8: STP O
HAPTER PERATION
Calculating the STP
Algorithm
Generating the
Configuration BPDU
Figure 1 Designated Switch and Designated Port
E1/0/1
E1/0/7
Switch B
E1/0/4
The following example illustrates the calculation process of STP.
The figure1-2 below illustrates the network.
Figure 2 Switch 7700 Networking
E1/0/7
Switch B
with priority 1
E1/0/4
To facilitate the descriptions, only the first four parts of the configuration BPDU are
given in the example. They are root ID (expressed as Ethernet switch priority), path
cost to the root, designated switch ID (expressed as Ethernet switch priority) and
the designated port ID (expressed as the port number). As illustrated in the figure
above, 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.
When initialized, each port of the switches will generate the configuration BPDU
taking themself as the root, root path cost as 0, designated switch IDs as their own
switch IDs and the designated ports as their ports.
Switch A
■
Configuration BPDU of Ethernet 1/0/1: {0, 0, 0, e1/0/1}
Configuration BPDU of Ethernet 1/0/2: {0, 0, 0, e1/0/2}
Switch B
■
Switch A
E1/0/2
E1/0/5
Switch C
E1/0/1
LAN
Switch A
with priority 0
E1/0/1
E1/0/2
5
10
4
E1/0/1
E1/0/5
Switch C
with priority 2