Cisco Catalyst 3750-X User Manual

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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-21521-01
Chapter 21 Configuring MSTP
Understanding MSTP
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 21-1 on page 21-5 compar
es the IEEE standard and the Cisco prestandard terminology.
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 spanni
ng-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, it decrements the received remaining hop count by
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.
The message-age and maximum-age information in the RSTP portion of the BPDU remain the same
th
roughout the region, and the same values are propagated by the region designated ports at the
boundary.
Ta ble 21-1 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

Cisco Catalyst 3750-X Specifications

General

Switching Capacity	160 Gbps
Stacking Bandwidth	64 Gbps
Max Stacking Units	9
Layer Support	Layer 2 and Layer 3
Form Factor	Rack-mountable
DRAM	256 MB
Flash Memory	64 MB
Model	Cisco Catalyst 3750-X Series
Ports	24 or 48 10/100/1000 Ethernet ports
Uplink Interfaces	4 SFP or 2 SFP+
Features	StackWise Plus, EnergyWise
Image	IP Base or IP Services
Operating Temperature	0 to 40°C
Operating Humidity	10 to 85% noncondensing