Spanning Tree Algorithm Configuration3-1233Configuring Global SettingsGlobal settings apply to the entire switch.Command Usage• Spanning Tree Protocol 9Uses RSTP for the internal state machine, but sends only 802.1D BPDUs. Thiscreates one spanning tree instance for the entire network. If multiple VLANs areimplemented on a network, the path between specific VLAN members may beinadvertently disabled to prevent network loops, thus isolating group members.When operating multiple VLANs, we recommend selecting the MSTP option.• Rapid Spanning Tree ProtocolRSTP supports connections to either STP or RSTP nodes by monitoring theincoming protocol messages and dynamically adjusting the type of protocolmessages the RSTP node transmits, as described below:- STP Mode – If the switch receives an 802.1D BPDU (i.e., STP BPDU) after aport’s migration delay timer expires, the switch assumes it is connected to an802.1D bridge and starts using only 802.1D BPDUs.- RSTP Mode – If RSTP is using 802.1D BPDUs on a port and receives an RSTPBPDU after the migration delay expires, RSTP restarts the migration delay timerand begins using RSTP BPDUs on that port.• Multiple Spanning Tree Protocol- To allow multiple spanning trees to operate over the network, you must configurea related set of bridges with the same MSTP configuration, allowing them toparticipate in a specific set of spanning tree instances.- A spanning tree instance can exist only on bridges that have compatible VLANinstance assignments.- Be careful when switching between spanning tree modes. Changing modesstops all spanning-tree instances for the previous mode and restarts the systemin the new mode, temporarily disrupting user traffic.Command AttributesBasic Configuration of Global Settings• Spanning Tree State – Enables/disables STA on this switch. (Default: Enabled)• Spanning Tree Type – Specifies the type of spanning tree used on this switch:- STP: Spanning Tree Protocol (IEEE 802.1D); i.e., when this option is selected,the switch will use RSTP set to STP forced compatibility mode).- RSTP: Rapid Spanning Tree (IEEE 802.1w); RSTP is the default.- MSTP: Multiple Spanning Tree (IEEE 802.1s)• Priority – Bridge priority is used in selecting the root device, root port, anddesignated port. The device with the highest priority becomes the STA root device.However, if all devices have the same priority, the device with the lowest MAC9. STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLANboundaries.
