Mstp; Mstp Features; Mstp Basic Concepts - H3C S7500E-X Configuration Manual

A port's link type determines the type of BPDUs the port sends.
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An access port sends STP BPDUs.
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A trunk or hybrid port sends STP BPDUs in VLAN 1 and sends PVST BPDUs in other VLANs.

MSTP

MSTP overcomes the following STP, RSTP, and PVST limitations:
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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.
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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.
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PVST limitations—Because each VLAN has its spanning tree, the amount of PVST BPDUs is
proportional to the number of VLANs on a trunk or hybrid port. When the trunk or hybrid port
permits too many VLANs, both resources and calculations for maintaining the VLAN spanning
trees increase dramatically. If a status change occurs on the trunk or hybrid port that permits
multiple VLANs, the device CPU will be overburdened with recalculating the affected spanning
trees. As a result, network performance is degraded.

MSTP features

Developed based on IEEE 802.1s, MSTP overcomes the limitations of STP, RSTP, and PVST. In
addition to supporting rapid network convergence, it allows data flows of different VLANs to be
forwarded along separate paths. This provides a better load sharing mechanism for redundant links.
MSTP provides the following features:
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MSTP divides a switched network into multiple regions, each of which contains multiple
spanning trees that are independent of one another.
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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.
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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.
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MSTP is compatible with STP and RSTP, and partially compatible with PVST.

MSTP basic concepts

Figure 21 shows a switched network that contains four MST regions, each MST region containing
four MSTP devices. Figure 22 shows the networking topology of MST region 3.
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