Cisco Systems 3560 사용자 설명서

 lists the interoperability and compatibility among the supported spanning-tree modes in a 

network.

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.

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+.

For more information on IEEE 802.1Q trunks, see 

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 VLAN-bridge spanning tree allows the bridge groups to form a spanning tree on top of the 
individual VLAN spanning trees to prevent loops from forming if there are multiple connections among 
VLANs. It also prevents the individual spanning trees from the VLANs being bridged from collapsing 
into a single spanning tree.

PVST+

Yes

Yes (with restrictions)

Yes (reverts to PVST+)

MSTP

Yes (with restrictions)

Yes

Yes (reverts to PVST+)

Rapid PVST+

Yes (reverts to PVST+)

Yes (reverts to PVST+)

Yes