1-9MSTP OverviewBackground of MSTPDisadvantages of STP and RSTPSTP does not support rapid state transition of ports. A newly elected root port or designated port mustwait twice the forward delay time before transiting to the forwarding state, even if it is a port on apoint-to-point link or it is an edge port (an edge port refers to a port that directly connects to a userterminal rather than to another device or a shared LAN segment.)The rapid spanning tree protocol (RSTP) is an optimized version of STP. RSTP allows a newly electedroot port or designated port to enter the forwarding state much quicker under certain conditions than inSTP. As a result, it takes a shorter time for the network to reach the final topology stability.z In RSTP, the state of a root port can transit fast under the following conditions: the old root port onthe device has stopped forwarding data and the upstream designated port has started forwardingdata.z In RSTP, the state of a designated port can transit fast under the following conditions: thedesignated port is an edge port or a port connected with a point-to-point link. If the designated portis an edge port, it can enter the forwarding state directly; if the designated port is connected with apoint-to-point link, it can enter the forwarding state immediately after the device undergoeshandshake with the downstream device and gets a response.RSTP supports rapid convergence. Like STP, it is of the following disadvantages: all bridges in a LANare on the same spanning tree; redundant links cannot be blocked by VLAN; the packets of all VLANsare forwarded along the same spanning tree.Features of MSTPThe multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP and RSTP. In additionto support for rapid network convergence, it also allows data flows of different VLANs to be forwardedalong their own paths, thus providing a better load sharing mechanism for redundant links.MSTP features the following:z MSTP supports mapping VLANs to MST instances (MSTIs) by means of a VLAN-to-MSTI mappingtable. MSTP introduces instance (integrates multiple VLANs into a set) and can bind multipleVLANs to an instance, thus saving communication overhead and improving resource utilization.z MSTP divides a switched network into multiple regions, each containing multiple spanning treesthat are independent of one another.z MSTP prunes a ring network into a network with tree topology, preventing packets from beingduplicated and forwarded in a network endlessly. Furthermore, it offers multiple redundant pathsfor forwarding data, and thus achieves load balancing for forwarding VLAN data.z MSTP is compatible with STP and RSTP.
