H3C Technologies H3C S12500 Series Switches - Manual (Page 75)

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• 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 transiting 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, and the packets of all VLANs are forwarded along the same
spanning tree, so redundant links cannot be blocked based on VLAN and load sharing among VLANs
cannot be implemented.
The number of PVST BPDUs generated grows with that of permitted VLANs on trunk ports. When the
status of a trunk port transits, 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 also provides a better load sharing mechanism for redundant
links by allowing data flows of different VLANs to be forwarded along separate paths. For more
information about VLANs, see "Configuring VLANs."
MSTP includes the following features:
• MSTP divides a switched network into multiple regions, each containing 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, avoiding proliferation and endless cycling of
packets in a loop network. In addition, it provides multiple redundant paths for data forwarding,
supporting load balancing of VLAN data.
• MSTP is compatible with STP and RSTP, but is incompatible with PVST.
MSTP basic concepts
Figure 19 shows a switched network that comprises four MST regions, each MST region comprising four
MSTP devices. Figure 20 shows the networking topology of MST region 3. This section describes some
basic concepts of MSTP.