Rstp; Rstp Protocol Frames - H3C S5560S-EI Series Configuration Manual

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If a path fails, the root port on this path no longer receives new configuration BPDUs and the old
configuration BPDUs will be discarded due to timeout. The device generates a configuration
BPDU with itself as the root and sends 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 cannot be propagated throughout the network
immediately. As a result, 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 might occur.
STP timers
The most important timing parameters in STP calculation are forward delay, hello time, and max age.
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Forward delay
Forward delay is the delay time for port state transition. By default, the forward delay is 15
seconds.
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
immediately, a temporary loop will likely occur.
The newly elected root ports or designated ports must go through the listening and learning
states before they transit to the forwarding state. This requires twice the forward delay time and
allows the new configuration BPDU to propagate throughout the network.
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Hello time
The device sends configuration BPDUs at the hello time interval to the neighboring devices to
ensure that the paths are fault-free. By default, the hello time is 2 seconds. If the device does
not receive configuration BPDUs within the timeout period, it recalculates the spanning tree.
The formula for calculating the timeout period is timeout period = timeout factor × 3 × hello time.
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Max age
The device uses the max age to determine whether a stored configuration BPDU has expired
and discards it if the max age is exceeded. By default, the max age is 20 seconds. In the CIST
of an MSTP network, the device uses the max age timer to determine whether a configuration
BPDU received by a port has expired. If it is expired, a new spanning tree calculation process
starts. The max age timer does not take effect on MSTIs.
If a port does not receive any configuration BPDUs within the timeout period, the port transits to the
listening state. The device will recalculate the spanning tree. It takes the port 50 seconds to transit
back to the forwarding state. This period includes 20 seconds for the max age, 15 seconds for the
listening state, and 15 seconds for the learning state.
To ensure a fast topology convergence, make sure the timer settings meet the following formulas:
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2 × (forward delay – 1 second) ≥ max age
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Max age ≥ 2 × (hello time + 1 second)

RSTP

RSTP achieves rapid network convergence by allowing a newly elected root port or designated port
to enter the forwarding state much faster than STP.

RSTP protocol frames

An RSTP BPDU uses the same format as an STP BPDU except that a Version1 length field is added
to the payload of RSTP BPDUs. The differences between an RSTP BPDU and an STP BPDU are as
follows:
• Protocol version ID—The value is 0x02 for RSTP.
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