Multiple Spanning-Tree Regions - Cisco Catalyst 2960 series Configuration Manual

time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence
time. You can use the hello keyword to override the automatically calculated hello time.
Related Topics
Configuring the Root Switch , on page 264
Restrictions for MSTP, on page 242
Bridge ID, Device Priority, and Extended System ID, on page 214

Multiple Spanning-Tree Regions

For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure the
switches with the same MST configuration information. A collection of interconnected switches that have the
same MST configuration comprises an MST region.
The MST configuration controls to which MST region each switch belongs. The configuration includes the
name of the region, the revision number, and the MST VLAN-to-instance assignment map. You configure
the switch for a region by specifying the MST region configuration on it. You can map VLANs to an MST
instance, specify the region name, and set the revision number. For instructions and an example, select the
"Specifying the MST Region Configuration and Enabling MSTP" link in Related Topics.
A region can have one or multiple members with the same MST configuration. Each member must be capable
of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number of MST regions in
a network, but each region can support up to 65 spanning-tree instances. Instances can be identified by any
number in the range from 0 to 4094. You can assign a VLAN to only one spanning-tree instance at a time.
Related Topics
Illustration of MST Regions, on page 248
Specifying the MST Region Configuration and Enabling MSTP , on page 261
Prerequisites for MSTP, on page 241
Restrictions for MSTP, on page 242
Spanning-Tree Interoperability and Backward Compatibility, on page 222
Optional Spanning-Tree Configuration Guidelines
BackboneFast, on page 294
UplinkFast, on page 290
IST, CIST, and CST
Unlike PVST+ and Rapid PVST+ in which all the spanning-tree instances are independent, the MSTP establishes
and maintains two types of spanning trees:
• An internal spanning tree (IST), which is the spanning tree that runs in an MST region.
Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a special
instance for a region, known as the internal spanning tree (IST). All other MST instances are numbered
from 1 to 4094.
The IST is the only spanning-tree instance that sends and receives BPDUs. All of the other spanning-tree
instance information is contained in M-records, which are encapsulated within MSTP BPDUs. Because
the MSTP BPDU carries information for all instances, the number of BPDUs that need to be processed
to support multiple spanning-tree instances is significantly reduced.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
Information About MSTP
245