Operation Manual – MSTPH3C S3100 Series Ethernet Switches Chapter 1 MSTP Configuration1-11Note:z In RSTP, the state of a root port can transit fast under the following conditions: theold root port on the device has stopped forwarding data and the upstreamdesignated port has started forwarding data.z In RSTP, the state of a designated port can transit fast under the followingconditions: the designated port is an edge port or a port connected with apoint-to-point link. If the designated port is an edge port, it can enter the forwardingstate directly; if the designated port is connected with a point-to-point link, it canenter the forwarding state immediately after the device undergoes handshake withthe downstream device and gets a response.RSTP supports rapid convergence. Like STP, it is of the following disadvantages: allbridges in a LAN are on the same spanning tree; redundant links cannot be blocked byVLAN; the packets of all VLANs are forwarded along the same spanning tree.II. Features of MSTPThe multiple spanning tree protocol (MSTP) overcomes the shortcomings of STP andRSTP. In addition to support for rapid network convergence, it also allows data flows ofdifferent VLANs to be forwarded along their own paths, thus providing a better loadsharing mechanism for redundant links.MSTP features the following:z MSTP supports mapping VLANs to MST instances by means of aVLAN-to-instance mapping table. MSTP introduces “instance” (integrates multipleVLANs into a set) and can bind multiple VLANs to an instance, thus savingcommunication overhead and improving resource utilization.z MSTP divides a switched network into multiple regions, each containing multiplespanning trees that are independent of one another.z MSTP prunes a ring network into a network with tree topology, preventing packetsfrom being duplicated and forwarded in a network endlessly. Furthermore, it offersmultiple redundant paths for forwarding data, and thus achieves load balancing forforwarding VLAN data.z MSTP is compatible with STP and RSTP.1.2.2 Basic MSTP TerminologiesFigure 1-4 illustrates basic MSTP terms (assuming that MSTP is enabled on eachswitch in this figure).
