Spanning Tree Operation Within An Mst Region; Spanning Tree Operations Between Mst Regions - Cisco Nexus 7000 Series Configuration Manual

Configuring MST Using Cisco NX-OS

Spanning Tree Operation Within an MST Region

The IST connects all the MSTdevices 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 devices at the boundary of the
region as the CIST regional root.
When an MST device 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 device
also initializes all of its MSTIs and claims to be the root for all of them. If the device receives superior MSTI
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
devices 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 devices in the MST region must agree on the same CIST regional root. Any two devices 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 devices 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 devices 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 device to adjacent STP devices and MST regions.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
IST, CIST, and CST
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