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Chapter 7
| Spanning Tree Algorithm
Configuring Global Settings for STA
Configuring Global Settings for STA
Use the Spanning Tree > STA (Configure Global - Configure) page to configure
global settings for the spanning tree that apply to the entire switch.
Command Usage
◆
Spanning Tree Protocol
This option uses RSTP set to STP forced compatibility mode. It 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
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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
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BPDU after the migration delay expires, RSTP restarts the migration delay
timer and begins using RSTP BPDUs on that port.
◆
Multiple Spanning Tree Protocol
MSTP generates a unique spanning tree for each instance. This provides
multiple pathways across the network, thereby balancing the traffic load,
preventing wide-scale disruption when a bridge node in a single instance fails,
and allowing for faster convergence of a new topology for the failed instance.
To allow multiple spanning trees to operate over the network, you must
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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
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VLAN instance assignments.
Be careful when switching between spanning tree modes. Changing
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modes stops all spanning-tree instances for the previous mode and restarts
the system in the new mode, temporarily disrupting user traffic.
2. STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLAN
boundaries.
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