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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter18 Configuring MSTP Understanding 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:
•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 ar e
numbered from 1 to 4094.
The IST is the only spanning-tree instan ce t h at s end s and r ec eives BP DU s. Al l o f th e ot her
spanning-tree instance information is contained in M-records, which are enc apsulated within MSTP
BPDUs. Because the MSTP BPDU carries information for all instances, the number o f 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 r egion 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 coll ection of the ISTs in each MST region,
and the common spanning tree (CST) that interconnects the MST regions and single spannin g trees.
The spanning tree computed in a region appears as a subtree in the CST that encompasses th e 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 page18-3 and the
“Operations Between MST Regions” section on page 18-4.
Note The implementation of the IEEE 802.1s standard, changes some of the terminology associ ated with MST
implementations. For a summary of these changes, see Table 17 -1 on page 17-5.

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 802.1 s
standard) as shown in Figure 18-1 on page18-4. 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 CIST r oot if there is only one region
in the network. If the CIST root is outside the region, one of the MSTP switches at the boun da ry 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 C IST 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 f ort h) t h an c urr en tly
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. Th us all s ubregions shrink, except for the one that conta in s the
true CIST regional root.