Ist, Cist, And Cst; Ist, Cist, And Cst Overview - Cisco Nexus 7000 Series Configuration Manual

IST, CIST, and CST

You must set the revision number when required as part of the MST configuration. The revision number
Note
is not incremented automatically each time that the MST configuration is committed.
• VLAN-to-MST instance mapping—4096-element table that associates each of the potential 4094 VLANs
When you change the VLAN-to-MSTI mapping, the system reconverges MST.
Note
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.
• The CST interconnects the MST regions and any instance of 802.1D and 802.1w STP that may be
• A CIST is a collection of the ISTs in each MST region. The CIST is the same as an IST inside an MST
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 devices 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.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
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supported in each virtual device context (VDC) 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.
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. Only CST information crosses region boundaries.
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.
region, and the same as a CST outside an MST region.
Configuring MST Using Cisco NX-OS