H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 167)

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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.
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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.
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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.
Introduction to MSTP
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.
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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.
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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.
Although RSTP support rapid network convergence, it has the same drawback as STP does: All bridges
within a LAN share the same spanning tree, so redundant links cannot be blocked based on VLANs,
and the packets of all VLANs are forwarded along the same spanning tree.
2) Features of MSTP
The multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP and RSTP. In addition
to support for rapid network convergence, it also allows data flows of different VLANs to be forwarded
along their own paths, thus providing a better load sharing mechanism for redundant links. For