Introduction to MSTP

MSTP overcomes the following STP and RSTP limitations:

•STP limitations—STP does not support rapid state transition of ports. A newly elected port must wait twice the forward delay time before it transits to the forwarding state, even if it connects to a point-to-point link or is an edge port.

•RSTP limitations—Although RSTP enables faster network convergence than STP, RSTP fails to provide load balancing among VLANs. As with STP, all RSTP bridges in a LAN share one spanning tree and forward packets from all VLANs along this spanning tree.

MSTP features

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.

MSTP provides the following features:

•MSTP divides a switched network into multiple regions, each of which contains multiple spanning trees that are independent of one another.

•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 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.

MSTP basic concepts

Figure 176 shows a switched network that comprises four MST regions, each MST region comprising four MSTP devices.

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