H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – MSTP
H3C S7500E Series Ethernet Switches Chapter 1 MSTP Configuration
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root port and designated port begin to forward data as soon as they are elected, a
temporary loop may occur.
3) STP timers
STP calculations need 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 will cause
re-calculation of the spanning tree, and the spanning tree structure will change
accordingly. However, the new configuration BPDU as the calculation result
cannot be propagated throughout the network immediately. If the newly elected
root port 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, a newly
elected root port or designated port requires twice the forward delay time before
transitioning to the forwarding state, when the new configuration BPDU has been
propagated throughout the network.
z Hello time is the time interval at which a device sends hello packets to the
surrounding devices to make sure that the paths are fault-free.
z Max age is a parameter used to determine whether a configuration BPDU held in
the device has expired. A configuration BPDU beyond the max age will be
discarded.
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, which
directly connects to a user terminal rather than to another device or a shared LAN
segment.
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.