User's Manual

MST Configuration Information
The MST configuration that must be identical on all switches within a single MST region is configured by
the user.
You can configure the following three parameters of the MST configuration:
• Name—32-character string, null padded and null terminated, identifying the MST region
• Revision number—Unsigned 16-bit number that identifies the revision of the current MST configuration
You must set the revision number when required as part of the MST configuration. The revision number
is not incremented automatically each time that the MST configuration is committed.
Note
• MST configuration table—4096-element table that associates each of the potential 4094 VLANs supported
to a given instance with the first (0) and last element (4095) set to 0. The value of element number X
represents the instance to which VLAN X is mapped.
When you change the VLAN-to-MSTI mapping, the system restarts MST.Caution
MST BPDUs contain these three configuration parameters. An MST bridge accepts an MST BPDU into its
own region only if these three configuration parameters match exactly. If one configuration attribute differs,
the MST bridge considers the BPDU to be from another MST region.
IST, CIST, and CST
IST, CIST, and CST Overview
Unlike Rapid PVST+, in which all the STP instances are independent, MST establishes and maintains IST,
CIST, and CST spanning trees, as follows:
•
An IST is the spanning tree that runs in an MST region.
MST establishes and maintains additional spanning trees within each MST region; these spanning trees are
called, multiple spanning tree instances (MSTIs).
Instance 0 is a special instance for a region, known as the IST. The IST always exists on all ports; you cannot
delete the IST, or Instance 0. By default, all VLANs are assigned to the IST. All other MST instances are
numbered from 1 to 4094.
The IST is the only STP instance that sends and receives BPDUs. All of the other MSTI information is
contained in MST records (M-records), which are encapsulated within MST BPDUs.
All MSTIs within the same region share the same protocol timers, but each MSTI has its own topology
parameters, such as the root bridge ID, the root path cost, and so forth.
An MSTI is local to the region; for example, MSTI 9 in region A is independent of MSTI 9 in region B, even
if regions A and B are interconnected.
Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
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Configuring Multiple Spanning Tree
MST Configuration Information