Supported Spanning-Tree Instances; Spanning-Tree Interoperability And Backward Compatibility - Cisco ME 3400 Software Configuration Manual

Understanding Spanning-Tree Features
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For information about the number of supported spanning-tree instances, see the next section.

Supported Spanning-Tree Instances

In PVST+ or rapid-PVST+ mode, the switch supports up to 128 spanning-tree instances.
In MSTP mode, the switch supports up to 65 MST instances. The number of VLANs that can be mapped
to a particular MST instance is unlimited.

Spanning-Tree Interoperability and Backward Compatibility

Table 14-2
network.
Table 14-2
PVST+
MSTP
Rapid PVST+
In a mixed MSTP and PVST+ network, the common spanning-tree (CST) root must be inside the MST
backbone, and a PVST+ switch cannot connect to multiple MST regions.
When a network contains switches running rapid PVST+ and switches running PVST+, we recommend
that the rapid-PVST+ switches and PVST+ switches be configured for different spanning-tree instances.
In the rapid-PVST+ spanning-tree instances, the root switch must be a rapid-PVST+ switch. In the
PVST+ instances, the root switch must be a PVST+ switch. The PVST+ switches should be at the edge
of the network.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
14-10
The rapid PVST+ uses the same configuration as PVST+ (except where noted), and the switch needs
only minimal extra configuration. The benefit of rapid PVST+ is that you can migrate a large PVST+
install base to rapid PVST+ without having to learn the complexities of the MSTP configuration and
without having to reprovision your network. In rapid-PVST+ mode, each VLAN runs its own
spanning-tree instance up to the maximum supported.
MSTP—This spanning-tree mode is based on the IEEE 802.1s standard. You can map multiple
VLANs to the same spanning-tree instance, which reduces the number of spanning-tree instances
required to support a large number of VLANs. The MSTP runs on top of the RSTP (based on IEEE
802.1w), which provides for rapid convergence of the spanning tree by eliminating the forward delay
and by quickly transitioning root ports and designated ports to the forwarding state. You cannot run
MSTP without RSTP.
The most common initial deployment of MSTP is in the backbone and distribution layers of a
Layer 2 switched network. For more information, see
lists the interoperability and compatibility among the supported spanning-tree modes in a
PVST+, MSTP , and Rapid-PVST+ Interoperability
PVST+
Yes
Yes (with restrictions)
Yes (reverts to PVST+)
Chapter 15, "Configuring MSTP."
MSTP
Yes (with restrictions)
Yes
Yes (reverts to PVST+)
Chapter 14 Configuring STP
Rapid PVST+
Yes (reverts to PVST+)
Yes (reverts to PVST+)
Yes
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