H3C Technologies H3C S5560 Series Switches - Manual (Page 103)

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PVST
In an STP- or RSTP-enabled LAN, all bridges share one spanning tree. Traffic from all VLANs is forwarded
along the spanning tree, and ports cannot be blocked on a per-VLAN basis to prune loops.
PVST allows every VLAN to have its own spanning tree, which increases utilization of links and
bandwidth. Because each VLAN runs STP or RSTP independently, a spanning tree only serves its VLAN.
A PVST-enabled H3C device can communicate with a third-party device that is running Rapid PVST or
PVST. The PVST-enabled H3C device supports fast network convergence like RSTP when connected to
PVST-enabled H3C devices or third-party devices enabled with Rapid PVST.
A port's link type determines the type of BPDUs the port sends.
• An access port sends STP BPDUs.
• 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:
• 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.
• 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.
• 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:
• 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, and partially compatible with PVST.