1-34The maximum hop count configured on the region roots of an MST region limits the size of the MSTregion.A configuration BPDU contains a field that maintains the remaining hops of the configuration BPDU.And a switch discards the configuration BPDUs whose remaining hops are 0. After a configurationBPDU reaches a root bridge of a spanning tree in a MST region, the value of the remaining hops field inthe configuration BPDU is decreased by 1 every time the configuration BPDU passes one switch. Sucha mechanism disables the switches that are beyond the maximum hops from participating in spanningtree calculation, and thus limits the size of an MST region.With such a mechanism, the maximum hops configured on the switch operating as the root bridge of theCIST or an MSTI in a MST region becomes the network diameter of the spanning tree, which limits thesize of the spanning tree in the current MST region. The switches that are not root bridges in an MSTregion adopt the maximum hop settings of the root bridge.Examples# Set the maximum hop count of the current MST region to 35. system-viewSystem View: return to User View with Ctrl+Z.[Sysname] stp max-hops 35stp mcheckSyntaxstp mcheckViewSystem view, Ethernet port viewParametersNoneDescriptionUse the stp mcheck command to perform the mCheck operation on the current port.When a port on an MSTP-enabled upstream switch connects with an STP-enabled downstream switch,the port operates in the STP-compatible mode automatically. But when the STP-enabled downstreamswitch is then replaced by an MSTP-enabled switch, the port cannot automatically transit to the MSTPmode but still remains in the STP-compatible mode. In this case, you can force the port to transit to theMSTP mode by performing the mCheck operation on the port.Similarly, when a port on an RSTP-enabled upstream switch connects with an STP-enableddownstream switch, the port operates in the STP-compatible mode. But when the STP-enableddownstream switch is then replaced by an MSTP-enabled switch, the port cannot automatically transitto the MSTP mode but remains in the STP-compatible mode. In this case, you can force the port totransit to the MSTP-compatible mode by performing the mCheck operation on the port.Related commands: stp mode, stp interface mcheck.Examples# Perform the mCheck operation on Ethernet 1/0/1.
