H3C S12500X-AF Series, Configuration Manual, Page: 98 / 224

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Basic concepts in PVST
PVST uses the same port roles and port states as RSTP for fast convergence. For more information,
see "
Basic concepts in RSTP ."
How PVST works
In PVST, each VLAN runs RSTP independently to maintain its own spanning tree without affecting
the spanning trees of other VLANs. In this way, loops in each VLAN are eliminated and traffic of
different VLANs is load shared over links. PVST uses RSTP BPDUs in the default VLAN and PVST
BPDUs in other VLANs for spanning tree calculation. H3C PVST implements per-VLAN spanning
tree calculation by mapping each VLAN to an MSTI.
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 frames 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 frames 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.
MSTP protocol frames
Figure 29 shows the format of an MSTP BPDU.