H3c-technologies H3C S5120 Series Switches User Manual download pdf (Page 173)

100

101

1-8

Figure 1-3 The final calculated spanning tree

AP1 AP2

Device A

With priority 0

Device B

With priority 1

Device C

With priority 2

BP1

BP2

CP2

The spanning tree calculation process in this example is only simplified process.

The BPDU forwarding mechanism in STP

z Upon network initiation, every switch regards itself as the root bridge, generates configuration

BPDUs with itself as the root, and sends the configuration BPDUs at a regular hello interval.

z If it is the root port that received a configuration BPDU and the received configuration BPDU is

superior to the configuration BPDU of the port, the device increases the message age carried in the

configuration BPDU following a certain rule and starts a timer to time the configuration BPDU while

sending out this configuration BPDU through the designated port.

z If the configuration BPDU received on a designated port has a lower priority than the configuration

BPDU of the local port, the port immediately sends out its own configuration BPDU in response.

z If a path becomes faulty, the root port on this path will no longer receive new configuration BPDUs

and the old configuration BPDUs will be discarded due to timeout. In this case, the device will

generate a configuration BPDU with itself as the root and send out the BPDUs and TCN BPDUs.

This triggers a new spanning tree calculation process to establish a new path to restore the

network connectivity.

However, the newly calculated configuration BPDU will not be propagated throughout the network

immediately, so the old root ports and designated ports that have not detected the topology change

continue forwarding data along the old path. If the new root ports and designated ports begin to forward

data as soon as they are elected, a temporary loop may occur.

STP timers

STP calculation involves three important timing parameters: forward delay, hello time, and max age.

z Forward delay is the delay time for device state transition.

A path failure can cause spanning tree re-calculation to adapt the spanning tree structure to the change.

However, the resulting new configuration BPDU cannot propagate throughout the network immediately.

If the newly elected root ports and designated ports start to forward data right away, a temporary loop is

likely to occur.

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