Operations Between Mst Regions; Ieee 802.1S Terminology - Cisco Catalyst 2960-XR Configuration Manual

Configuring Multiple Spanning-Tree Protocol
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Illustration of MST Regions, on page 48

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
related to the spanning-tree topology (for example, switch priority, port VLAN cost, and port VLAN priority)
can be configured on both the CST instance and the MST instance.
MSTP switches use Version 3 RSTP BPDUs or IEEE 802.1D STP BPDUs to communicate with legacy IEEE
802.1D switches. MSTP switches use MSTP BPDUs to communicate with MSTP switches.
Related Topics
Illustration of MST Regions, on page 48

IEEE 802.1s Terminology

Some MST naming conventions used in Cisco's prestandard implementation have been changed to identify
some internal or regional parameters. These parameters are significant only within an MST region, as opposed
to external parameters that are relevant to the whole network. Because the CIST is the only spanning-tree
instance that spans the whole network, only the CIST parameters require the external rather than the internal
or regional qualifiers.
• The CIST root is the root switch for the unique instance that spans the whole network, the CIST.
• The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an MST
region. Remember that an MST region looks like a single switch for the CIST. The CIST external root
path cost is the root path cost calculated between these virtual switches and switches that do not belong
to any region.
• The CIST regional root was called the IST master in the prestandard implementation. If the CIST root
is in the region, the CIST regional root is the CIST root. Otherwise, the CIST regional root is the closest
switch to the CIST root in the region. The CIST regional root acts as a root switch for the IST.
• The CIST internal root path cost is the cost to the CIST regional root in a region. This cost is only relevant
to the IST, instance 0.
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Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
IST, CIST, and CST
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