Introduction To Mstp - H3C LS-3100-52P-OVS-H3 Operation Manual

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 MSTP

Why MSTP
1) Weakness 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
point-to-point link or 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 converge.
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.
Although RSTP supports 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
VLAN, 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 the support for rapid network convergence, it also allows data flows of different VLANs to be
forwarded along separate paths, thus providing a better load sharing mechanism for redundant links.
For description about VLANs, refer to VLAN Configuration in the Access Volume.
MSTP features the following:
MSTP supports mapping VLANs to MST instances (MSTIs) by means of a VLAN-to-MSTI mapping
table. MSTP can reduce communication overheads and resource usage by mapping multiple
VLANs to one MSTI.
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