3Com Switch 8800 Advanced Software V5 Configuration Guide

102 CHAPTER 11: MSTP CONFIGURATION
Introduction to MSTP Why MSTP
1 Disadvantages 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 transitioning to the
forwarding state, even if it is a port on a point-to-point link or it is an edge port.
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 reach the final topology stability.
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■ 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 (a port is an edge
port if it is not connected to the other devices directly or indirectly) 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 support 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 VLANs, 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 support for rapid network convergence, it also allows data
flows of different VLANs to be forwarded along their own paths, thus providing a
better load sharing mechanism for redundant links. For description about VLANs,
refer to
“VLAN Overview” on page 155.
MSTP features the following:
■ MSTP supports mapping VLANs to MST instances by means of a
VLAN-to-instance mapping table;
■ 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 network with tree topology. As a network
of this type is loop-free, it prevents packets in it from being duplicated and
forwarded endlessly. In addition, MSTP can provide multiple redundant paths
for data forwarding , thus allowing for load balancing in VLANs;
■ MSTP is compatible with STP and RSTP.
Some concepts in MSTP
As shown in Figure 28, there are four multiple spanning tree (MST) regions, each
made up of four switches running MSTP. In light with the diagram, the following
paragraphs will present some concepts of MSTP.