H3C Technologies H3C S10500 Series Switches - Manual (Page 79)

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PVST
PVST was introduced to improve link bandwidth usage in network environments where multiple virtual
LANs (VLANs) exist. Unlike STP and RSTP whose bridges in a LAN must forward their VLAN packets in
the same spanning tree, PVST allows each VLAN to build a separate spanning tree.
PVST uses the following BPDUs:
• STP BPDUs: Sent by access ports according to the VLAN status, or by trunk ports and hybrid ports
according to the status of VLAN 1.
• PVST BPDUs: Sent by trunk port and hybrid ports according to the status of permitted VLANs except
VLAN 1.
MSTP
STP, RSTP, and PVST 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.
Although RSTP supports rapid network convergence, it has the same drawback as STP—All bridges
within a LAN share the same spanning tree, so redundant links cannot be blocked based on VLAN, and
the packets of all VLANs are forwarded along the same spanning tree.
The number of PVST BPDUs generated grows with that of permitted VLANs on trunk ports. When the
status of a trunk port transitions, network devices might be overloaded to re-calculate a large number of
spanning trees.
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 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 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 divides a switched network into multiple regions, each of which contains multiple spanning
trees that are independent of one another.
• 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, but is incompatible with PVST.
MSTP basic concepts
Figure 20 shows a switched network that comprises four MST regions, each MST region comprising four
MSTP devices. Figure 21 shows the networking topology of MST region 3.