Introduction To Mstp - H3C S9500 Series Operation Manual

Operation Manual – MSTP
H3C S9500 Series Routing Switches
However, the newly computed 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 through the old path. If the new
root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur.
3) STP timers
STP calculation involves three timing parameters: forward delay, hello time, and max
age.
Forward delay is the delay for port state transition. A path failure can cause
re-calculation of the spanning tree, and hence topology change. As it takes time
for the new configuration BPDU to propagate throughout the network, forwarding
data on the root port or designated port immediately upon its selection can cause a
temporary loop. To avoid this, STP defines that a newly elected root port or
designated port must wait twice the forward delay time before it begins to forward
data. The delay ensures that the new configuration BPDU has been propagated
throughout the network.
Hello time is the time interval at which a device sends hello packets to the adjacent
devices to ensure that the paths are fault-free.
Max age determines how long a configuration BPDU can be held by the device.
The device discards a configuration BPDU once its age reaches the max age.

1.1.2 Introduction to MSTP

I. Why MSTP
1) Disadvantages 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 transitioning to the
forwarding state, even if it is a port on a point-to-point link or it is an edge port.
The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows a
newly elected root port or designated port to enter the forwarding state much quicker
under certain conditions than in STP. As a result, it takes a shorter time for the network
to reach the final topology stability.
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Chapter 1 MSTP Configuration