Supported Spanning-Tree Instances; Spanning-Tree Interoperability And Backward Compatibility; Stp And Ieee 802.1Q Trunks - Cisco Catalyst 3750 Software Configuration Manual

Understanding Spanning-Tree Features

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 16 MST instances. The number of VLANs that can be mapped
to a particular MST instance is unlimited.
For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP), see the
"Spanning-Tree Configuration Guidelines" section on page

Spanning-Tree Interoperability and Backward Compatibility

Table 14-2
network.
Table 14-2 PVST+, MSTP , and Rapid-PVST+ Interoperability
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.

STP and IEEE 802.1Q Trunks

The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strategy for
a network. The standard requires only one spanning-tree instance for all VLANs allowed on the trunks.
However, in a network of Cisco switches connected through 802.1Q trunks, the switches maintain one
spanning-tree instance for each VLAN allowed on the trunks.
When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch uses
PVST+ to provide spanning-tree interoperability. If rapid PVST+ is enabled, the switch uses it instead
of PVST+. The switch combines the spanning-tree instance of the 802.1Q VLAN of the trunk with the
spanning-tree instance of the non-Cisco 802.1Q switch.
However, all PVST+ or rapid-PVST+ information is maintained by Cisco switches separated by a cloud
of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as
a single trunk link between the switches.
PVST+ is automatically enabled on 802.1Q trunks, and no user configuration is required. The external
spanning-tree behavior on access ports and Inter-Switch Link (ISL) trunk ports is not affected by
PVST+.
For more information on 802.1Q trunks, see
Catalyst 3750 Metro Switch Software Configuration Guide
14-10
lists the interoperability and compatibility among the supported spanning-tree modes in a
PVST+
Yes
Yes (with restrictions)
Yes (reverts to PVST+)
14-12.
MSTP
Yes (with restrictions)
Yes
Yes (reverts to PVST+)
Chapter 10, "Configuring VLANs."
Chapter 14 Configuring STP
Rapid PVST+
Yes (reverts to PVST+)
Yes (reverts to PVST+)
Yes
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