Operation Manual – MSTPH3C S7500E Series Ethernet Switches Chapter 1 MSTP Configuration1-11root port and designated port begin to forward data as soon as they are elected, atemporary loop may occur.3) STP timersSTP calculations need three important timing parameters: forward delay, hello time,and max age.z Forward delay is the delay time for device state transition. A path failure will causere-calculation of the spanning tree, and the spanning tree structure will changeaccordingly. However, the new configuration BPDU as the calculation resultcannot be propagated throughout the network immediately. If the newly electedroot port and designated ports start to forward data right away, a temporary loop islikely to occur. For this reason, as a mechanism for state transition in STP, a newlyelected root port or designated port requires twice the forward delay time beforetransitioning to the forwarding state, when the new configuration BPDU has beenpropagated throughout the network.z Hello time is the time interval at which a device sends hello packets to thesurrounding devices to make sure that the paths are fault-free.z Max age is a parameter used to determine whether a configuration BPDU held inthe device has expired. A configuration BPDU beyond the max age will bediscarded.1.1.2 Introduction to MSTPI. Why MSTP1) Disadvantages of STP and RSTPSTP does not support rapid state transition of ports. A newly elected root port ordesignated port must wait twice the forward delay time before transitioning to theforwarding state, even if it is a port on a point-to-point link or it is an edge port, whichdirectly connects to a user terminal rather than to another device or a shared LANsegment.The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows anewly elected root port or designated port to enter the forwarding state much quickerunder certain conditions than in STP. As a result, it takes a shorter time for the networkto reach the final topology stability.
