H3C Technologies H3C SecPath F1000-E - Manual (Page 201)

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generate configuration BPDUs with itself as the root and send out the 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.
• 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.
For this reason, as a mechanism for state transition in STP, the newly elected root ports or designated
ports require twice the forward delay time before transiting to the forwarding state to ensure that the new
configuration BPDU has propagated throughout the network.
• Hello time is the time interval at which a device sends hello packets to the surrounding devices to
ensure that the paths are fault-free.
• Max age is a parameter used to determine whether a configuration BPDU held by the device has
expired. A configuration BPDU beyond the max age will be discarded.
Introduction to RSTP
Developed based on the 802.1w standard of IEEE, RSTP is an optimized version of STP. It achieves rapid
network convergence by allowing a newly elected root port or designated port to enter the forwarding
state much quicker under certain conditions than in STP.
NOTE:
•
In RSTP, a newly elected root port can enter the forwarding state rapidly if this condition is met: the old
root port on the device has stopped forwarding data and the upstream designated port has started
forwarding data.
• In RSTP, a newly elected designated port can enter the forwarding state rapidly if this condition is met:
the designated port is an edge port or a port connected with a point-to-point link. If the designated port
is an edge port, it can enter the forwarding state directly; if the designated port is connected with a
point-to-point link, it can enter the forwarding state immediately after the device undergoes handshake
with the downstream device and gets a response.
Introduction to MSTP
Why MSTP
Weaknesses of STP and RSTP
STP does not support rapid state transition of ports. A newly elected root port or designated port must
wait twice the forward delay time before transiting to the forwarding state, even if it is a port on a