Ist, Cist, And Cst; Operations Within An Mst Region - Cisco Catalyst 2950 Software Configuration Manual

Chapter 14 Configuring MSTP

IST, CIST, and CST

Unlike PVST+ and rapid PVST+ in which all the spanning-tree instances are independent, the MSTP
establishes and maintains two types of spanning trees:
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For more information, see the
"Operations Between MST Regions" section on page

Operations Within an MST Region

The IST connects all the MSTP switches in a region. When the IST converges, the root of the IST
becomes the IST master , which is the switch within the region with the lowest switch ID and path cost
to the CST root. The IST master also is the CST root if there is only one region within the network. If
the CST root is outside the region, one of the MSTP switches at the boundary of the region is selected
as the IST master.
When an MSTP switch initializes, it sends BPDUs claiming itself as the root of the CST and the IST
master, with both of the path costs to the CST root and to the IST master set to zero. The switch also
initializes all of its MST instances and claims to be the root for all of them. If the switch receives superior
MST root information (lower switch ID, lower path cost, and so forth) than currently stored for the
switch, it relinquishes its claim as the IST master.
During initialization, a region might have many subregions, each with its own IST master. As switches
receive superior IST information, they leave their old subregions and join the new subregion that might
contain the true IST master. Thus all subregions shrink, except for the one that contains the true IST
master.
For correct operation, all switches in the MST region must agree on the same IST master. Therefore, any two
switches in the region only synchronize their port roles for an MST instance if they converge to a common
IST master.
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An internal spanning tree (IST), which is the spanning tree that runs in an MST region.
Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a
special instance for a region, known as the internal spanning tree (IST). All other MST instances are
numbered from 1 to 15.
The IST is the only spanning-tree instance that sends and receives BPDUs. All of the other
spanning-tree instance information is contained in M-records, which are encapsulated within MSTP
BPDUs. Because the MSTP 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 switch 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 common and internal spanning tree (CIST), which is a collection of the ISTs in each MST region,
and the common spanning tree (CST) that interconnects the MST regions and single spanning trees.
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 switches that
support the IEEE 802.1w, IEEE 802.1s and IEEE 802.1D standards. The CIST inside an MST region
is the same as the CST outside a region.
"Operations Within an MST Region" section on page 14-3
14-4.
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
Understanding MSTP
and the
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