Configuring Global Settings
Global settings apply to the entire switch.
Command Usage
• Spanning Tree Protocol9
Uses RSTP for the internal state machine, but sends only 802.1D BPDUs. This
creates one spanning tree instance for the entire network. If multiple VLANs are
implemented on a network, the path between specific VLAN members ma y be
inadvertently disabled to prevent network loops, thus isolating group members.
When operating multiple VLANs, we recommend selecting the MSTP option.
• Rapid Spanning Tree Protocol9
RSTP supports connections to either STP or RSTP nod es by monit oring the
incoming protocol messages and dynamically adjusting the type of protocol
messages the RSTP node transmits, as described below:
- STP Mode – If the switch receives an 802.1D BP DU (i.e ., STP BP DU) after a
port’s migration delay timer expires, the switch assumes it is connect ed to an
802.1D bridge and starts using only 802.1D BPDUs.
- RSTP Mode – If RSTP is using 802.1D BPDUs on a port and receives an RST P
BPDU after the migration delay expires, RSTP restarts the migration delay timer
and begins using RSTP BPDUs on that port.
• Multiple Spanning Tree Protocol
- To allow multiple spanning trees to operate over the network, you must configu re
a related set of bridges with the same MSTP configuration , allowing the m to
participate in a specific set of spanning tree instances.
- A spanning tree instance can exist only on bridges that have compatib le VLAN
instance assignments.
- Be careful when s witc hing b etwee n spann ing tree mode s. C hanging mo des
stops all spanning-tree instances for the previous mode and restar ts th e syste m
in the new mode, temporarily disrupting user traffic.
Command Attributes
Basic Configuration of Global Settings
• Spa nning Tree State – Enables/disables STA on this switch. (Default: Enabled)
• Spanning Tree Type – Specifies the ty pe of span ning tree us ed 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 Spannin g Tree (IEEE 802 .1w); RST P is the defaul t.
- MSTP: Multiple Spanning Tre e (IEEE 802 .1s)
• Priority – Bridge priority is used in selecting the root devi ce, root po rt, and
designated 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 MA C
9. STP and RSTP BPDUs are transmitted as untag ged fra mes, and will cro ss any VLA N
boundaries.

10-6

Spanning Tree Algorithm

10