Cisco nexus 5000 series Cli Configuration Manual page 148

Information About MST
S e n d f e e d b a c k t o n x 5 0 0 0 - d o c f e e d b a c k @ c i s c o . c o m
Figure 1-2
Legacy 802.1D
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:
•
•
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
1-6
MST Regions, CIST Regional Roots, and CST Root
D
B CIST Regional
Root
MST Region 2
The CIST root is the root bridge for the CIST, which is the unique instance that spans the whole
network.
The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an
MST region. An MST region looks like a single switch to the CIST. The CIST external root path
cost is the root path cost calculated between these virtual switches and switches that do not belong
to any region.
CIST Regional
A
Root and CST root
MST Region 1
Chapter 1 Configuring MST
CIST Regional
C
Root
MST Region 3
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