User's Manual

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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.
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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.
Spanning Tree Operation Within an MST Region
The IST connects all the MST switches in a region. When the IST converges, the root of the IST becomes the
CIST regional root. The CIST regional root is also the CIST root if there is only one region in the network.
If the CIST root is outside the region, the protocol selects one of the MST switches at the boundary of the
region as the CIST regional root.
When an MST switch initializes, it sends BPDUs that identify itself as the root of the CIST and the CIST
regional root, with both the path costs to the CIST root and to the CIST regional root set to zero. The switch
also initializes all of its MSTIs and claims to be the root for all of them. If the switch receives superior MST
root information (lower switch ID, lower path cost, and so forth) than the information that is currently stored
for the port, it relinquishes its claim as the CIST regional root.
During initialization, an MST region might have many subregions, each with its own CIST regional root. As
switches receive superior IST information from a neighbor in the same region, they leave their old subregions
and join the new subregion that contains the true CIST regional root. This action causes all subregions to
shrink except for the subregion that contains the true CIST regional root.
All switches in the MST region must agree on the same CIST regional root. Any two switches in the region
will only synchronize their port roles for an MSTI if they converge to a common CIST regional root.
Spanning Tree Operations Between MST Regions
If you have multiple regions or 802.1 w or 802.1D STP instances within a network, MST establishes and
maintains the CST, which includes all MST regions and all 802.1w and 802.1D STP switches in the network.
The MSTIs combine with the IST at the boundary of the region to become the CST.
The IST connects all the MST switches in the region and appears as a subtree in the CIST that encompasses
the entire switched domain. The root of the subtree is the CIST regional root. The MST region appears as a
virtual switch to adjacent STP switches and MST regions.
Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
92 OL-26590-01
Configuring Multiple Spanning Tree
IST, CIST, and CST