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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 (a port that
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.
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 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, and the packets from all VLANs are forwarded along the same
spanning tree, so redundant links cannot be blocked based on VLAN and traffic cannot be load-shared
among VLANs.
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 provides a better load sharing mechanism for redundant links
by allowing data flows of different VLANs to be forwarded along separate paths.
