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Chapter 17
Configuring MSTP
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.
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.
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Table 17-1 on page 17-5
Table 17-1
IEEE Standard
CIST regional root
CIST internal root path cost
CIST external root path cost
MSTI regional root
MSTI internal root path cost
Hop Count
The IST and MST instances do not use the message-age and maximum-age information in the
configuration BPDU to compute the spanning-tree topology. Instead, they use the path cost to the root
and a hop-count mechanism similar to the IP time-to-live (TTL) mechanism.
By using the spanning-tree mst max-hops global configuration command, you can configure the
maximum hops inside the region and apply it to the IST and all MST instances in that region. The hop
count achieves the same result as the message-age information (triggers a reconfiguration). The root
switch of the instance always sends a BPDU (or M-record) with a cost of 0 and the hop count set to the
OL-8915-03
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.
compares the IEEE standard and the Cisco prestandard terminology.
Prestandard and Standard Terminology
Cisco Prestandard
IST master
IST master path cost
Root path cost
Instance root
Root path cost
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
Understanding MSTP
Cisco Standard
CIST regional root
CIST internal path cost
Root path cost
Instance root
Root path cost
17-5
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Troubleshooting
