C
HAPTER
4
| Configuring the Switch
Configuring the Spanning Tree Algorithm
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â—† Rapid Spanning Tree Protocol
1
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:
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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.
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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.
â—† 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.
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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.
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A spanning tree instance can exist only on bridges that have
compatible VLAN instance assignments.
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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.
PARAMETERS
These parameters are displayed:
Basic Settings
◆ Protocol Version – Specifies the type of spanning tree used on this
switch. (Options: STP, RSTP, MSTP; Default: MSTP)
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STP: Spanning Tree Protocol (IEEE 802.1D); i.e., the switch will use
RSTP set to STP forced compatibility mode.
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RSTP: Rapid Spanning Tree (IEEE 802.1w)
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MSTP: Multiple Spanning Tree (IEEE 802.1s); This is the default.
1. STP and RSTP BPDUs are transmitted as untagged frames, and will cross any VLAN
boundaries.
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