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272 Rockwell Automation Publication 1783-UM007I-EN-P - December 2017
Chapter 3 Configure Switch Features
Spanning Tree Protocol (STP) STP, the IEEE 802.1D bridge protocol, is a Layer 2 link management protocol
that provides path redundancy and helps to prevent loops in the network. The
switch supports the following STP versions:
•
Multiple Spanning Tree Protocol (MSTP) based on the IEEE 802.1s
standard.
MSTP uses Rapid Spanning Tree Protocol (RSTP) for rapid
convergence. This mode maps a group of VLANs into a single spanning
tree instance, with each instance having a spanning tree topology
independent of other spanning tree instances. This architecture provides
multiple forwarding paths for data traffic, enables load balancing, and
reduces the number of spanning tree instances required to support a
large number of VLANs. MSTP is the default STP mode.
•
Per VLAN Spanning Tree Plus (PVST+) protocol based on the
IEEE 802.1D standard.
PVST+ runs on each VLAN on the switch up to the maximum
supported, to help create a loop-free path through the network. PVST+
provides Layer 2 load balancing for the VLAN on which it runs. You can
create different logical topologies by using the VLANs on your network
to make sure that all of your links are used but that no one link is
oversubscribed. Each instance of PVST+ on a VLAN has a single root
switch. This root switch propagates the spanning-tree information
associated with that VLAN to all other switches in the network. Because
each switch has the same information about the network, this process
maintains the network topology.
•
Rapid per VLAN Spanning Tree Plus (Rapid PVST+) protocol based
on the IEEE 802.1w standard.
RPVST+ is the same as PVST+ except that is uses a rapid convergence
based on the IEEE 802.1w standard. To provide rapid convergence, the
rapid PVST+ immediately deletes dynamically learned MAC ID entries
on a per-port basis upon receiving a topology change. By contrast,
PVST+ uses a short aging time for dynamically learned MAC ID
entries. Only one version can be active on the switch at any time. For
example, all VLANs run PVST+, all VLANs run rapid PVST+, or all
VLANs run MSTP.
In MSTP mode, the switch supports a maximum of 65 MST instances. The
number of VLANs that can be mapped to a particular MST instance is
unlimited.
In PVST+ or rapid-PVST+ mode, the switch supports a maximum of 128
spanning tree instances.