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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Software Configuration Guide—Release 8.2GLX
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Chapter 7 Configuring Spanning Tree
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 that is 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 that is 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
per port 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 that is 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 MISTP instance has a single root switch, which 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.