Specifications

9-10
Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
OL-26895-01
Chapter 9 Configuring STP
Configuring Spanning-Tree Features
Supported Spanning-Tree Instances
In PVST+ mode, the switch supports up to 128 spanning-tree instances.
In MSTP mode, the switch supports up to 65 MST instances. You can map up to 255 VLANs to a
particular MST instance.
Spanning-Tree Interoperability and Backward Compatibility
Table 9-2 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.
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 MWR 2941 to a non-Cisco device through an IEEE 802.1Q trunk, the router
uses PVST+ to provide spanning-tree interoperability.
All 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 IEEE 802.1Q trunks, and no user configuration is required. The
external spanning-tree behavior on access ports is not affected by PVST+.
For more information on IEEE 802.1Q trunks, see Chapter 7, “Configuring VLANs.”
Configuring Spanning-Tree Features
• Default Spanning-Tree Configuration, page 9-11
• Spanning-Tree Configuration Guidelines, page 9-11
• Enabling Spanning Tree on an ENI, page 9-12 (required)
• Disabling Spanning Tree, page 9-14 (optional)
• Configuring the Router as a Root Switch, page 9-14 (optional)
• Configuring a Secondary Root Switch, page 9-16 (optional)
Table 9-2 PVST+ and MSTP Interoperability
PVST+ MSTP
PVST+ Yes Yes (with restrictions)
MSTP Yes (with restrictions) Yes