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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 21 Configuring MSTP Understanding MSTP
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.
For more information, see the “Operations Within an MST Region” section on page 21-3 and the
“Operations Between MST Regions” section on page 21-3.
Note The implementation of the IEEE 802.1s standard, changes some of the terminology associated with MST
implementations. For a summary of these changes, see Table 20-1 on page 20-5.
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) as shown in Figure 21-1 on page 21-4. 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 C IST roo t 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. Thus 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.
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.