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 248
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.
Table 30: Prestandard and Standard Terminology
Cisco StandardCisco PrestandardIEEE Standard
CIST regional rootIST masterCIST regional root
CIST internal path costIST master path costCIST internal root path cost
Root path costRoot path costCIST external root path cost
Instance rootInstance rootMSTI regional root
Root path costRoot path costMSTI internal root path cost
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
247
Information About MSTP
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