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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 faster under certain conditions than STP.
NOTE:
In RSTP, a newly elected root port can enter the forwarding state rapidly if 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 the designated port is an edge
port (a port that directly connects to a user terminal rather than to another device or a shared LAN segment) 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
Why MSTP
Limitations 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 it transits to the forwarding state, even if it is a port on a point-
to-point link or an edge port.
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.
Features of MSTP
Developed based on IEEE 802.1s, MSTP overcomes the limitations of STP and RSTP. In addition to
supporting for rapid network convergence, it provides a better load sharing mechanism for redundant
links by allowing data flows of different VLANs to be forwarded along separate paths. For more
information about VLANs, see the chapter ―VLAN configuration.‖
MSTP provides the following features:
MSTP supports mapping VLANs to spanning tree instances by means of a VLAN-to-instance
mapping table. MSTP can reduce communication overheads and resource usage by mapping
multiple VLANs to one instance.
MSTP divides a switched network into multiple regions, each of which contains multiple spanning
trees that are independent of one another.
MSTP prunes a loop network into a loop-free tree, which avoids proliferation and endless cycling of
packets in a loop network. In addition, it supports load balancing of VLAN data by providing
multiple redundant paths for data forwarding.
MSTP is compatible with STP and RSTP.
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