S
PANNING
T
REE
C
OMMANDS
4-208
Command Usage
• Spanning Tree Protocol
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
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 after a port’s
migration delay timer expires, the switch assumes that 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.
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