3Com 4210G Series, Configuration Manual, Page: 78 / 1133

4-10
Introduction to MSTP
Why MSTP
Weaknesses 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.
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.
Features of MSTP
Developed based on IEEE 802.1s, MSTP overcomes the shortcomings of STP and RSTP. In addition
to the support for rapid network convergence, it 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:
z
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.
z
MSTP divides a switched network into multiple regions, each containing multiple spanning trees
that are independent of one another.
z
MSTP prunes a loop network into a loop-free tree, thus avoiding proliferation and endless cycling
of packets in a loop network. In addition, it provides multiple redundant paths for data forwarding,
thus supporting load balancing of VLAN data.
z
MSTP is compatible with STP and RSTP.