Cisco Catalyst 2960 Series, Configuration Manual

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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
The IST connects all the MSTP switches in a region. When the IST converges, the root of the IST becomes
the CIST regional root (called the
IST master before the implementation of the IEEE 802.1s standard). It is
the switch within the region with the lowest switch ID and path cost to the CIST root. The CIST regional root
is also the CIST root if there is only one region in the network. If the CIST root is outside the region, one of
the MSTP switches at the boundary of the region is selected as the CIST regional root.
When an MSTP switch initializes, it sends BPDUs claiming itself as the root of the CIST and the CIST regional
root, with both of the path costs to the CIST root and to the CIST regional root 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 port, it
relinquishes its claim as the CIST regional root.
During initialization, a region might have many subregions, each with its own CIST regional root. As switches
receive superior IST information, they leave their old subregions and join the new subregion that contains the
true CIST regional root. All subregions shrink except for the one that contains the true CIST regional root.
For correct operation, all switches in the MST region must agree on the same CIST regional root. Therefore,
any two switches in the region only synchronize their port roles for an MST instance if they converge to a
common CIST regional root.
Related Topics
Illustration of MST Regions, on page 248
Operations Between MST Regions
If there are multiple regions or legacy IEEE 802.1D switches within the network, MSTP establishes and
maintains the CST, which includes all MST regions and all legacy STP switches in the network. The MST
instances combine with the IST at the boundary of the region to become the CST.
The IST connects all the MSTP switches in the region and appears as a subtree in the CIST that encompasses
the entire switched domain. The root of the subtree is the CIST regional root. The MST region appears as a
virtual switch to adjacent STP switches and MST regions.
Only the CST instance sends and receives BPDUs, and MST instances add their spanning-tree information
into the BPDUs to interact with neighboring switches 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
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
246
Information About MSTP