21-3
Cisco IE 2000 Switch Software Configuration Guide
OL-25866-01
Chapter 21 Configuring MSTP
Information About Configuring MSTP
• 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 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 associ ated with MST
implementations. For a summary of these changes, see Table 20-1 on page 20-4.
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 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 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 dary 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 orth) 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 f or 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 t he 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.
Figure 21-1 shows a network with three MST regions and a legacy IEEE 802.1D sw itch (D). The CIST
regional root for region 1 (A) is also the CIST root. The CIST regional roo t for region 2 (B) and the CIST
regional root for region 3 (C) are the roots for their respective subtrees within the CIST. The RSTP runs
in all regions.