Pvst+ Mode; Rapid Pvst; Mistp Mode - Cisco WS-C2950SX-48-SI Configuration Manual

Understanding How PVST+ and MISTP Modes Work
The following sections provide an overview of each mode.
Caution If your network currently uses PVST+ and you plan to use MISTP on any switch, you must first enable
MISTP-PVST+ on the switch and configure an MISTP instance to avoid causing network loops.

PVST+ Mode

PVST+ is the default STP used on all Ethernet, Fast Ethernet, and Gigabit Ethernet port-based VLANs
on Catalyst 4500 series switches. PVST+ runs on each VLAN on the switch, ensuring that each has 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 using the VLANs on your network to ensure that all of your links will be used but no one link
will be 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 knowledge about the network, this process ensures that the network topology is maintained.

Rapid PVST+

Rapid PVST+ is the same as PVST+, except that Rapid PVST+ utilizes a Rapid STP based on IEEE
802.1w instead of 802.1D. Rapid PVST+ uses the same configuration as PVST+, and you need only
minimal extra configuration. With Rapid PVST+, dynamic CAM entries are flushed immediately on a
per-port basis upon any topology change. UplinkFast and BackboneFast are enabled but not active in
this mode, because the functionality is built into the rapid STP. This method provides for quick recovery
of connectivity following the failure of a bridge, bridge port, or LAN.

MISTP Mode

MISTP is an optional STP that runs on Catalyst 4500 series switches. MISTP allows you to group
multiple VLANs under a single instance of spanning tree (an MISTP instance). MISTP combines the
Layer 2 load-balancing benefits of PVST+ with the lower CPU load of IEEE 802.1Q.
An MISTP instance is a virtual logical topology defined by a set of bridge and port parameters; an
MISTP instance becomes a real topology when VLANs are mapped to it. Each MISTP instance has its
own root switch and a different set of forwarding links (that is different bridge and port parameters).
Each instance of MISTP has a single root switch. This root switch propagates the information that is
associated with that instance of MISTP to all other switches in the network. This process ensures that
the network topology is maintained because each switch has the same knowledge about the network.
MISTP builds MISTP instances by exchanging MISTP BPDUs with peer entities in the network. There
is only one BPDU for each MISTP instance, rather than for each VLAN as in PVST+. There are fewer
BPDUs in an MISTP network; therefore, there is less overhead in the network. MISTP discards any
PVST+ BPDUs that it sees.
An MISTP instance can have any number of VLANs that are mapped to it, but a VLAN can only be
mapped to a single MISTP instance. You can easily move a VLAN (or VLANs) in an MISTP topology
to another MISTP instance if it has converged. (However, if ports are added at the same time the VLAN
is moved, convergence time is required.)
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
7-12
Chapter 7 Configuring Spanning Tree
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Release 8.1
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