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 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.
54
