Mst Terminology - Cisco Nexus 7000 Series Configuration Manual

Configuring MST Using Cisco NX-OS
IST, CIST, and CST
This figure shows a network with three MST regions and an 802.1D device (D). The CIST regional root for
region 1 (A) is also the CIST root. The CIST regional root for region 2 (B) and the CIST regional root for
region 3 (C) are the roots for their respective subtrees within the CIST.
Figure 13: MST Regions, CIST Regional Roots, and CST Root
Only the CST instance sends and receives BPDUs. MSTIs add their spanning tree information into the BPDUs
(as M-records) to interact with neighboring devices within the same MST region and compute the final
spanning tree topology. The spanning tree parameters related to the BPDU transmission (for example, hello
time, forward time, max-age, and max-hops) are configured only on the CST instance but affect all MSTIs.
You can configure the parameters related to the spanning tree topology (for example, the switch priority, the
port VLAN cost, and the port VLAN priority) on both the CST instance and the MSTI.
MST devices use Version 3 BPDUs. If the MST device falls back to 802.1D STP, the device uses only 802.1D
BPDUs to communicate with 802.1D-only devices. MST devices use MST BPDUs to communicate with
MST devices.

MST Terminology

MST naming conventions include identification of some internal or regional parameters. These parameters
are used only within an MST region, compared to external parameters that are used throughout the whole
network. Because the CIST is the only spanning tree instance that spans the whole network, only the CIST
parameters require the external qualifiers and not the internal or regional qualifiers. The MST terminology is
as follows:
• The CIST root is the root bridge for the CIST, which is the unique instance that spans the whole network.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
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