H3C Technologies H3C WX5500E Series Access Controllers - Manual (Page 72)

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The device uses the max age to determine whether a stored configuration BPDU has expired and
discards it if the max age is exceeded.
RSTP
RSTP achieves rapid network convergence by allowing a newly elected root port or designated port to
enter the forwarding state much faster than STP.
A newly elected RSTP root port rapidly enters the forwarding state if the old root port on the device has
stopped forwarding data and the upstream designated port has started forwarding data.
A newly elected RSTP designated port rapidly enters the forwarding state if it is an edge port (which
directly connects to a user terminal rather than to another network device or a shared LAN segment), or
it connects to a point-to-point link (to another device). Edge ports directly enter the forwarding state.
When connecting to a point-to-point link, a designated port enters the forwarding state immediately after
the device receives a handshake response from the directly connected device.
PVST
PVST was introduced to improve link bandwidth usage in network environments where multiple 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 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
This section describes the fundamentals of 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 transitions 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 packets from 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 the number of permitted VLANs on trunk ports. When
the status of a trunk port transitions, network devices might be overloaded with recalculating a large
number of spanning trees.