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| 3.5 |  | Evolution of Spanning Tree |
| 3.5.2 |  | Enhanced PVST + |

MISTP Mode
After the development of PVST+, someone got the idea that it would be useful to have an STP mode that allows for a compromise between PVST+ and MST. Multiple Instance of Spanning-Tree Protocol (MISTP), allows the grouping of multiple VLANs under a single instance of spanning tree. MISTP combines the Layer 2 load-balancing benefits of PVST+ with the lower CPU load of IEEE 802.1Q.

MISTP is an optional STP mode that runs on Catalyst 4000 and 6000 family switches. A MISTP instance is a virtual logical topology defined by a set of bridge and port parameters. A 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). This Root Switch propagates the information 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 a MISTP network, so there is less over-head in the network. MISTP discards any PVST+ BPDUs it sees.

A MISTP instance can have any number of VLANs mapped to it, but a VLAN can be mapped to only a single MISTP instance. VLAN(s) can be easily moved in a 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.

MISTP-PVST+ Mode
MISTP-PVST+ is a transition STP mode that allows the use of MISTP functionality of Catalyst 4000 and 6000 series switches while continuing to communicate with the older Catalyst switches in the network that use PVST+. A switch using PVST+ mode and a switch using MISTP mode that are connected cannot see the BPDUs of the other switch, a condition that can cause loops in the network. MISTP-PVST+ allows interoperability between PVST+ and MISTP, because it detects the BPDUs of both modes. To convert a network to MISTP, MISTP-PVST+ can be used to transition the network from PVST+ to MISTP in order to avoid problems. MISTP-PVST+ conforms to the limits of PVST+.