Calculation Process Of The Stp Algorithm - H3C SR8800 Configuration Manual

Calculation process of the STP algorithm

The STP algorithm uses the following calculation process:
Initial state
1.
Upon initialization of a router, each port generates a BPDU with the router as the root bridge, in
which the root path cost is 0, designated bridge ID is the device ID, and the designated port is the
port itself.
Selection of the root bridge
2.
Initially, each STP-enabled router 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 routers compare their
root bridge IDs to elect the router with the smallest root bridge ID as the root bridge.
Selection of the root port and designated ports on a non-root router
3.
Table 2
Table 2 Selection of the root port and designated ports
Step
1
2
3
NOTE:
When the network topology is stable, only the root port and designated ports forward user traffic, while
other ports are all in the blocked state to receive BPDUs but not forward BPDUs or user traffic.
Table 3 Selection of the optimum configuration BPDU
Step
1
describes the process of selecting the root port and designated ports.
Description
A non-root-bridge router regards the port on which it received the optimum configuration BPDU as
the root port. Table 3 describes how the optimum configuration BPDU is selected.
Based on the configuration BPDU and the path cost of the root port, the router calculates a
designated port configuration BPDU for each of the rest ports.
•
The root bridge ID is replaced with that of the configuration BPDU of the root port.
•
The root path cost is replaced with that of the configuration BPDU of the root port plus the path
cost of the root port.
•
The designated bridge ID is replaced with the ID of this router.
•
The designated port ID is replaced with the ID of this port.
The router compares the calculated configuration BPDU with the configuration BPDU on the port
whose port role is to be determined:
•
If the calculated configuration BPDU is superior, the router considers this port as the designated
port, replaces the configuration BPDU on the port with the calculated configuration BPDU, and
periodically sends out the calculated configuration BPDU.
•
If the configuration BPDU on the port is superior, the router blocks this port without updating its
configuration BPDU. The blocked port can receive BPDUs, but cannot send BPDUs or forward
data traffic.
Actions
Upon receiving a configuration BPDU on a port, the router compares the priority of the received
configuration BPDU with that of the configuration BPDU generated by the port, and:
•
If the former priority is lower, the router discards the received configuration BPDU and does not
process the configuration BPDU of this port.
•
If the former priority is higher, the router replaces the content of the configuration BPDU
generated by the port with the content of the received configuration BPDU.
28