User's Manual

The following figure shows a network with three MST regions and an 802.1D switch (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 15: 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 switches and compute the final spanning tree topology. Because
of this, 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 switches use Version 3 BPDUs or 802.1D STP BPDUs to communicate with 802.1D-only switches.
MST switches use MST BPDUs to communicate with MST switches.
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 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
OL-26590-01 93
Configuring Multiple Spanning Tree
IST, CIST, and CST