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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter18 Configuring MSTP Understanding MSTP
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 communica te with M STP switc hes.

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 whol e ne twork . Beca use the C IST i s
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. O ther wise , t he C IST region al roo t 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.
Table18-1 on pa ge 18-5 compares the IEEE standard and the Cisco prestandard terminology.
Hop Count
The IST and MST instances do not use the message-age and maximum-age informat ion in the
configuration BPDU to compute the spanning-tree topology. Instead, they use the path cost to the r oot
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
maximum value. When a switch receives this BPDU, i t de crem ents th e rec ei v ed r emain ing hop co unt b y
one and propagates this value as the remaining hop count in the BPDUs it generates. When the count
reaches zero, the switch discards the BPDU and ages the information held for the port.
Table18-1 Prestandard and Standard Terminology
IEEE Standard Cisco Prestandard Cisco Standard
CIST regional root IST master CIST regional root
CIST internal root path cost IST master path cost CIST internal path cost
CIST external root path cost Root path cost Root path cost
MSTI regional root Instance root Instance root
MSTI internal root path cost Root path cost Root path cost