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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.
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 to 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 to 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
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.
Although RSTP supports rapid network convergence, it has the same drawback as STP—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.
MSTP features
Developed based on IEEE 802.1s, MSTP overcomes the limitations 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, providing a better load sharing mechanism for redundant links.
MSTP includes the following features:
• MSTP supports mapping VLANs to MST instances (MSTIs) by means of a VLAN-to-instance
mapping table. MSTP can reduce communication overheads and resource usage by mapping
multiple VLANs to one MSTI.
• MSTP divides a switched network into multiple regions, each containing multiple spanning trees
that are independent of one another.
• MSTP prunes a loop network into a loop-free tree, avoiding proliferation and endless cycling of
packets in a loop network. In addition, it provides multiple redundant paths for data forwarding,
supporting load balancing of VLAN data.
• MSTP is compatible with STP and RSTP.
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