Ist, Cist, And Cst - Cisco 7604 Configuration Manual
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Chapter 19
Configuring Standard-Compliant IEEE MST
A region can have one or multiple members with the same MST configuration; each member must be
capable of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number of MST
regions in a network, but each region can support up to 65 spanning tree instances. Instances can be
identified by any number in the range from 0 to 4094. You can assign a VLAN to only one spanning tree
instance at a time.
IST, CIST, and CST
These sections describe internal spanning tree (IST), common and internal spanning tree (CIST), and
common spanning tree (CST):
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IST, CIST, and CST Overview
Unlike other spanning tree protocols, in which all the spanning tree instances are independent, MST
establishes and maintains IST, CIST, and CST spanning trees:
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The spanning tree computed in a region appears as a subtree in the CST that encompasses the entire
switched domain. The CIST is formed by the spanning tree algorithm running among routers that support
the 802.1w, 802.1s, and 802.1D standards. The CIST inside an MST region is the same as the CST
outside a region.
For more information, see the
and the
OL-4266-08
IST, CIST, and CST Overview, page 19-3
Spanning Tree Operation Within an MST Region, page 19-4
Spanning Tree Operations Between MST Regions, page 19-4
IEEE 802.1s Terminology, page 19-5
An IST is the spanning tree that runs in an MST region.
Within each MST region, MST maintains multiple spanning tree instances. Instance 0 is a special
instance for a region, known as the IST. All other MST instances are numbered from 1 to 4094.
The IST is the only spanning tree instance that sends and receives BPDUs. All of the other
spanning tree instance information is contained in MSTP records (M-records), which are
encapsulated within MST BPDUs. Because the MST BPDU carries information for all instances, the
number of BPDUs that need to be processed to support multiple spanning tree instances is
significantly reduced.
All MST instances within the same region share the same protocol timers, but each MST instance
has its own topology parameters, such as root bridge ID, root path cost, and so forth. By default, all
VLANs are assigned to the IST.
An MST instance is local to the region; for example, MST instance 1 in region A is independent of
MST instance 1 in region B, even if regions A and B are interconnected.
A CIST is a collection of the ISTs in each MST region.
The CST interconnects the MST regions and single spanning trees.
"Spanning Tree Operation Within an MST Region" section on page 19-4
"Spanning Tree Operations Between MST Regions" section on page
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Understanding MST
19-4.
19-3
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