Spanning-Tree Interoperability And Backward Compatibility; Stp And Ieee 802.1Q Trunks; Vlan-Bridge Spanning Tree - Cisco Catalyst 2960-XR Configuration Manual

Spanning Tree Protocol

Spanning-Tree Interoperability and Backward Compatibility

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.
All stack members run the same version of spanning tree (all PVST+, all Rapid PVST+, or all MSTP).
Table 5: PVST+, MSTP, and Rapid-PVST+ Interoperability and Compatibility
PVST+
MSTP
Rapid PVST+
Related Topics
Specifying the MST Region Configuration and Enabling MSTP, on page 59
MSTP Configuration Guidelines, on page 44
Multiple Spanning-Tree Regions, on page 45

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 IEEE 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 IEEE 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 IEEE 802.1Q VLAN of the trunk with the
spanning-tree instance of the non-Cisco IEEE 802.1Q switch.
However, all PVST+ or Rapid PVST+ information is maintained by Cisco switches separated by a cloud of
non-Cisco IEEE 802.1Q switches. The non-Cisco IEEE 802.1Q cloud separating the Cisco switches is treated
as a single trunk link between the switches.
PVST+ is automatically enabled on IEEE 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+.

VLAN-Bridge Spanning Tree

Cisco VLAN-bridge spanning tree is used with the fallback bridging feature (bridge groups), which forwards
non-IP protocols such as DECnet between two or more VLAN bridge domains or routed ports. The
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
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PVST+ MSTP
Yes Yes (with restrictions)
Yes (with restrictions) Yes
Yes (reverts to PVST+) Yes (reverts to PVST+)
Configuring Spanning Tree Protocol
Rapid PVST+
Yes (reverts to PVST+)
Yes (reverts to PVST+)
Yes
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