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PANNING
T
REE
A
LGORITHM
C
ONFIGURATION
3-163
Configuring Global Settings
Global settings apply to the entire switch.
Command Usage
• Spanning Tree Protocol
14
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
14
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.
14. STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLAN boundaries.