S
PANNING

T

REE

A

LGORITHM

12-8

Configuring Global Settings
Global settings apply to the entire switch.
Command Usage
• Spanning Tree Protocol13
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 disab led to prevent
network loops, thus isolating group members. When operating multiple
VLANs, we recommend selecting the MSTP option.
• Rapid Spanning Tree Protocol13
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.
13. STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLAN
boundaries.