58How MSTP worksMSTP divides an entire Layer 2 network into multiple MST regions, which are connected by a calculatedCST. Inside an MST region, multiple spanning trees are calculated. Each spanning tree is called an"MSTI." Among these MSTIs, MSTI 0 is the IST.Like STP, MSTP uses configuration BPDUs to calculate spanning trees. An important difference is that anMSTP BPDU carries the MSTP configuration of the bridge from which the BPDU is sent.CIST calculationThe calculation of a CIST tree is also the process of configuration BPDU comparison. During this process,the device with the highest priority is elected as the root bridge of the CIST. MSTP generates an IST withineach MST region through calculation. At the same time, MSTP regards each MST region as a singledevice and generates a CST among these MST regions through calculation. The CST and ISTs constitutethe CIST of the entire network.MSTI calculationWithin an MST region, MSTP generates different MSTIs for different VLANs based on theVLAN-to-instance mappings. For each spanning tree, MSTP performs a separate calculation processsimilar to spanning tree calculation in STP. For more information, see "Calculation process of the STPalgorithm."In MSTP, a VLAN packet is forwarded along the following paths:• Within an MST region, the packet is forwarded along the corresponding MSTI.• Between two MST regions, the packet is forwarded along the CST.Implementation of MSTP on devicesMSTP is compatible with STP and RSTP. Devices that run MSTP can recognize STP and RSTP protocolpackets used for spanning tree calculation.In addition to basic MSTP functions, the following functions are provided for ease of management:• Root bridge hold• Root bridge backup• Root guard• BPDU guard• Loop guard• TC-BPDU guard• BPDU dropProtocols and standards• IEEE 802.1d, Media Access Control (MAC) Bridges• IEEE 802.1w, Part 3: Media Access Control (MAC) Bridges—Amendment 2: Rapid Reconfiguration• IEEE 802.1s, Virtual Bridged Local Area Networks—Amendment 3: Multiple Spanning Trees
