S
PANNING
T
REE
A
LGORITHM
C
ONFIGURATION
3-155
Configuring Global Settings
Global settings apply to the entire switch.
Command Usage
• Spanning Tree Protocol
8
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 (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.
8.
STP and RSTP BPDUs are transmitted as untagged frames, and will cross any
VLAN boundaries.
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