53The device uses the max age to determine whether a stored configuration BPDU has expired anddiscards it if the max age is exceeded.RSTPRSTP achieves rapid network convergence by allowing a newly elected root port or designated port toenter 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 hasstopped 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 (whichdirectly connects to a user terminal rather than to another network device or a shared LAN segment) orit 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 thedevice receives a handshake response from the directly connected device.PVSTPVST was introduced to improve link bandwidth usage in network environments where multiple virtualLANs (VLANs) exist. Unlike STP and RSTP whose bridges in a LAN must forward their VLAN packets inthe 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 portsaccording to the status of VLAN 1.• PVST BPDUs—Sent by trunk port and hybrid ports according to the status of permitted VLANsexcept VLAN 1.MSTPSTP, RSTP, and PVST limitationsSTP does not support rapid state transition of ports. A newly elected port must wait twice the forwarddelay time before it transitions to the forwarding state, even if it connects to a point-to-point link or is anedge port.Although RSTP supports rapid network convergence, it has the same drawback as STP. All bridges withina LAN share the same spanning tree, and the packets of all VLANs are forwarded along the samespanning tree, so redundant links cannot be blocked based on VLAN and load sharing among VLANscannot be implemented.The number of PVST BPDUs generated grows with that of permitted VLANs on trunk ports. When thestatus of a trunk port transitions, network devices might be overloaded to re-calculate a large number ofspanning trees.
