65However, the newly calculated configuration BPDU cannot be propagated throughout the networkimmediately. As a result, the old root ports and designated ports that have not detected the topologychange continue forwarding data along the old path. If the new root ports and designated ports beginto forward data as soon as they are elected, a temporary loop might occur.STP timersThe most important timing parameters in STP calculation are forward delay, hello time, and max age.• Forward delayForward delay is the delay time for port state transition.A path failure can cause spanning tree re-calculation to adapt the spanning tree structure to thechange. However, the resulting new configuration BPDU cannot propagate throughout thenetwork immediately. If the newly elected root ports and designated ports start to forward dataimmediately, a temporary loop will likely occur.The newly elected root ports or designated ports require twice the forward delay time beforethey transit to the forwarding state. This allows the new configuration BPDU to propagatethroughout the network.• Hello timeThe device sends hello packets at the hello time interval to the neighboring devices to makesure the paths are fault-free.• Max ageThe device uses the max age to determine whether a stored configuration BPDU has expiredand discards it if the max age is exceeded.RSTPRSTP achieves rapid network convergence by allowing a newly elected root port or designated portto enter the forwarding state much faster than STP.A newly elected RSTP root port rapidly enters the forwarding state when the following conditionsexist:• The old root port on the device has stopped forwarding data.• 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 or itconnects to a point-to-point link.• A port that directly connects to a user terminal can be configured as an edge port. Edge portsdirectly enter the forwarding state.• When a designated port connects to a point-to-point link, it enters the forwarding stateimmediately after the device receives a handshake response from the directly connecteddevice.PVSTIn an STP- or RSTP-enabled LAN, all bridges share one spanning tree. Traffic from all VLANs isforwarded along the spanning tree, and ports cannot be blocked on a per-VLAN basis to prune loops.PVST allows every VLAN to have its own spanning tree, which increases usage of links andbandwidth. Because each VLAN runs STP or RSTP independently, a spanning tree only serves itsVLAN.A PVST-enabled H3C device can communicate with a third-party device that is running Rapid PVSTor PVST. The PVST-enabled H3C device supports fast network convergence like RSTP whenconnected to PVST-enabled H3C devices or third-party devices enabled with Rapid PVST.
