Table of Contents
Advertisement
Understanding PVST+ and MISTP Modes
If your network currently uses PVST+ and you plan to use MISTP on any switch, you must first enable
Caution
MISTP-PVST+ on the switch and configure a MISTP instance to avoid causing loops in the network.
PVST+ Mode
PVST+ is the default Spanning Tree Protocol used on all Ethernet, Fast Ethernet, and Gigabit Ethernet
port-based VLANs on Catalyst 4000 family 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.
MISTP Mode
MISTP is an optional spanning tree protocol that runs on Catalyst 4000 family switches. MISTP allows
you to group multiple VLANs under a single instance of spanning tree (a MISTP instance). MISTP
combines the Layer 2 load-balancing benefits of PVST+ with the lower CPU load of IEEE 802.1Q.
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).
Each instance of MISTP has a single root switch. 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; therefore, there is less overhead in the network. MISTP discards any
PVST+ BPDUs that it sees.
A MISTP instance can have any number of VLANs mapped to it, but a VLAN can only be mapped to a
single MISTP instance. You can easily move a VLAN (or VLANs) 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 spanning tree mode that allows you to use the MISTP functionality on
Catalyst 4000 family switches while continuing to communicate with the older Catalyst 5000 and 6000
switches in your network that use PVST+. A switch using PVST+ mode and a switch using MISTP mode
connected together cannot see the BPDUs of the other switch, a condition that can cause loops in the
network. MISTP-PVST+ allows interoperability between PVST+ and pure MISTP, because it detects the
BPDUs of both modes. If you wish to convert your network to MISTP, you can use MISTP-PVST+ to
transition the network from PVST+ to MISTP in order to avoid problems.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
7-12
Chapter 7
Configuring Spanning Tree
78-12647-02
Hide quick links:
Advertisement
Table of Contents
Troubleshooting
