Cisco 7604 Configuration Manual page 251

Chapter 19 Configuring Standard-Compliant IEEE MST
Figure 19-1 MST Regions, CIST Regional Roots, and CST Root
Legacy 802.1D
Only the CST instance sends and receives BPDUs, and MST instances add their spanning tree
information into the BPDUs to interact with neighboring routers and compute the final spanning tree
topology. Because of this, the spanning tree parameters related to BPDU transmission (for example,
hello time, forward time, max-age, and max-hops) are configured only on the CST instance but affect all
MST instances. Parameters related to the spanning tree topology (for example, switch priority, port
VLAN cost, and port VLAN priority) can be configured on both the CST instance and the MST instance.
MST routers use Version 3 BPDUs or 802.1D STP BPDUs to communicate with 802.1D routers. MST
routers use MST BPDUs to communicate with MST routers.
IEEE 802.1s Terminology
Some MST naming conventions used in the prestandard implementation have been changed to include
identification of some internal and 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.
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•
OL-4266-08
D
CIST Regional
B
Root
MST Region 2
The CIST root is the root bridge for the 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. Remember that an MST region looks like a single router to the CIST. The CIST
external root path cost is the root path cost calculated between these virtual routers and routers that
do not belong to any region.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
CIST Regional
A
Root and CST root
MST Region 1
Understanding MST
CIST Regional
C
Root
MST Region 3
19-5