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12-8
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 12 Configuring RSTP and MSTP
Understanding MSTP
IST, CIST, and CST
Unlike PVST+ in which all the spanning-tree instances are independent, the MSTP establishes and
maintains two types of spanning-trees:
• 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 by a switch 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 as a result of the spanning-tree algorithm running between
switches that support the 802.1W, 802.1S, and 802.1D protocols. The CIST inside an MST region
is the same as the CST outside a region.
For more information, see the “Operations Within an MST Region” section on page 12-8 and the
“Operations Between MST Regions” section on page 12-9.
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 (shown in Figure 12-3 on page 12-9), which is the switch within the region with
the lowest bridge 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 bridge ID, lower path cost, and so forth) than currently stored for the port,
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 synchronize their port roles for an MST instance only if they converge to a common
IST master.