Operation Manual – MSTPH3C S3100 Series Ethernet Switches Chapter 1 MSTP Configuration1-17z For configuration BPDUs with both the same Root bridge ID and the sameExternal path costs, Master bridge ID, Internal path cost, Designated bridge ID, IDof sending port, ID of receiving port are compared in turn.For MSTP, MSTI configuration information is generally expressed as follows:(Instance bridge ID, Internal path costs, Designated bridge ID, ID of sending port, ID ofreceiving port), so the compared as followsz The smaller the Instance bridge ID of the configuration BPDU is, the higher thepriority of the configuration BPDU is.z For configuration BPDUs with the same Instance bridge IDs, Internal path costsare compared.z For configuration BPDUs with both the same Instance bridge ID and the sameInternal path costs, Designated bridge ID, ID of sending port, ID of receiving portare compared in turn.3) A spanning tree is calculated as follows:z Determining the root bridgeRoot bridges are selected by configuration BPDU comparing. The switch with thesmallest root ID is chosen as the root bridge.z Determining the root portFor each switch in a network, the port on which the configuration BPDU with the highestpriority is received is chosen as the root port of the switch.z Determining the designated portFirst, the switch calculates a designated port configuration BPDU for each of its portsusing the root port configuration BPDU and the root port path cost, with the root IDbeing replaced with that of the root port configuration BPDU, root path cost beingreplaced with the sum of the root path cost of the root port configuration BPDU and thepath cost of the root port, the ID of the designated bridge being replaced with that of theswitch, and the ID of the designated port being replaced with that of the port.The switch then compares the calculated configuration BPDU with the originalconfiguration BPDU received from the corresponding port on another switch. If thelatter takes precedence over the former, the switch blocks the local port and keeps theport's configuration BPDU unchanged, so that the port can only receive configurationmessages and cannot forward packets. Otherwise, the switch sets the local port to thedesignated port, replaces the original configuration BPDU of the port with thecalculated one and advertises it regularly.1.2.4 MSTP Implementation on SwitchesMSTP is compatible with both STP and RSTP. That is, MSTP-enabled switches canrecognize the protocol packets of STP and RSTP and use them for spanning treecalculation. In addition to the basic MSTP functions, H3C series switches also providethe following functions for users to manage their switches.
