Loading ...
Loading ...
Loading ...
If the root bridge ID and the root path cost of X equal those of Y, but the bridge ID of X is
smaller than that of Y, X is superior to Y.
If the root bridge ID, the root path cost and bridge ID of X equal those of Y, but the port ID of X
is smaller than that of Y, X is superior to Y.
STP Generation
In the beginning
In the beginning, each switch regards itself as the root, and generates a configuration BPDU for
each port on it as a root, with the root path cost being 0, the ID of the designated bridge being
that of the switch, and the designated port being itself.
Comparing BPDUs
Each switch sends out configuration BPDUs and receives a configuration BPDU on one of its
ports from another switch. The following table shows the comparing operations.
Step
Operation
1
If the priority of the BPDU received on the port is lower than that of the BPDU if
of the port itself, the switch discards the BPDU and does not change the BPDU
of the port.
2 If the pr
iority of the BPDU is higher than that of the BPDU of the port itself, the
switch replaces the BPDU of the port with the received one and compares it
with those of other ports on the switch to obtain the one with the highest
priority.
Table 7-1 Comparing BPDUs
Selecting the root bridge
The root bridge is selected by BPDU comparing. The switch with the smallest root ID is chosen
as the root bridge.
Selecting the root port and designate port
The operation is taken in the following way:
Step
Operation
1
For each switch (except the one chosen as the root bridge) in a network, the
port that receives the BPDU with the highest priority is chosen as the root port
of the switch.
2 Using the root port BPDU and the root path co
st, the switch generates a
designated port BPDU for each of its ports.
Root ID is replaced with that of the root port;
Root path is replaced with the sum of the root path cost of the root port
and the path cost between this port and the root port;
The ID of the designated bridge is replaced with that of the switch;
The ID of the designated port is replaced with that of the port.
72
Loading ...
Loading ...
Loading ...