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Spanning Tree Algorithm Configuration
3-141
3
Note: The current root port and current root cost display as zero when this device is not
connected to the network.
Configuring Global Settings
Global settings apply to the entire switch.
Command Usage
• Spanning Tree Protocol
9
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 may be
inadvertently disabled to prevent network loops, thus isolating group members.
When operating multiple VLANs, we recommend selecting the MSTP option.
• Rapid Spanning Tree Protocol
9
RSTP supports connections to either STP or RSTP nodes by monitoring 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 BPDU (i.e., STP BPDU) after a
port’s migration delay timer expires, the switch assumes it is connected 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 RSTP
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 configure
a related set of bridges with the same MSTP configuration, allowing them to
participate in a specific set of spanning tree instances.
- A spanning tree instance can exist only on bridges that have compatible VLAN
instance assignments.
- Be careful when switching between spanning tree modes. Changing modes
stops all spanning-tree instances for the previous mode and restarts the system
in the new mode, temporarily disrupting user traffic.
Command Attributes
Basic 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);
9. STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLAN
boundaries.