Cisco N5010P-N2K-BE, Software Configuration Manual

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Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
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Chapter 9 Configuring MST
Information About MST
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
These sections describe internal spanning tree (IST), common and internal spanning tree (CIST), and
common spanning tree (CST):
•
IST, CIST, and CST Overview, page 9-4
• Spanning Tree Operation Within an MST Region, page 9-5
•
Spanning Tree Operations Between MST Regions, page 9-5
• MST Terminology, page 9-6
IST, CIST, and CST Overview
Unlike Rapid PVST+ (see Chapter 8, “Configuring Rapid PVST+” for more information on this subject),
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.
• The CST interconnects the MST regions and any instance of 802.1D and 802.1w STP that may be
running on the network. The CST is the one STP instance for the entire bridged network and
encompasses all MST regions and 802.1w and 802.1D instances.
•
A CIST is a collection of the ISTs in each MST region. The CIST is the same as an IST inside an
MST region, and the same as a CST outside an MST region.
The spanning tree computed in an MST region appears as a subtree in the CST that encompasses the
entire switched domain. The CIST is formed by the spanning tree algorithm running among switches that
support the 802.1w, 802.1s, and 802.1D standards. The CIST inside an MST region is the same as the
CST outside a region.
For more information, see the “Spanning Tree Operation Within an MST Region” section on page 9-5
and the “Spanning Tree Operations Between MST Regions” section on page 9-5 .