Nokia ESB26 User Manual page 153

14.
independent of MST instance 1 in region B, even if regions A and B are interconnected.
• CIST - A common and internal spanning tree, 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 calculated 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.
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 14-1), which is the switch within the region
with the lowest bridge ID and path cost to the CST root. The IST master is also 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.
Operation between MSTP Regions
If there are multiple regions or legacy 802.1D switches within the network, MSTP establishes
and maintains the CST, which includes all MST regions and all legacy STP switches in the
MN700004 Rev 01
Multiple Spanning Tree Protocol (MSTP)
Figure 14-1 MSTP Within a Region
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