Operation Manual – MSTPH3C S3100 Series Ethernet Switches Chapter 1 MSTP Configuration1-10root port and designated port begin to forward data as soon as they are elected, atemporary loop may occur.7) STP timersThe following three time parameters are important for STP calculation:z Forward delay, the period a device waits before state transition.A link failure triggers a new round of spanning tree calculation and results in changes ofthe spanning tree. However, as new configuration BPDUs cannot be propagatedthroughout the network immediately, if the new root port and designated port begin toforward data as soon as they are elected, loops may temporarily occur.For this reason, the protocol uses a state transition mechanism. Namely, a newlyelected root port and the designated ports must go through a period, which is twice theforward delay time, before they transit to the forwarding state. The period allows thenew configuration BPDUs to be propagated throughout the entire network.z Hello time, the interval for sending hello packets. Hello packets are used to checklink state.A switch sends hello packets to its neighboring devices at a regular interval (the hellotime) to check whether the links are faulty.z Max time, lifetime of the configuration BPDUs stored in a switch. A configurationBPDU that has “expired” is discarded by the switch.1.2 MSTP Overview1.2.1 Background of MSTPI. 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 transiting to theforwarding state, even if it is a port on a point-to-point link or it is an edge port (an edgeport refers to a port that directly connects to a user terminal rather than to anotherdevice or a shared LAN segment.)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.
