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Management Guide
CLI
TigerAccess™ EE
SMC7824M/VSW
267
Here explains how MSTP/PVSTP differently operates on the LAN. Suppose to configure
100 VLANs from SWITCH A to B and C. In case of STP, there is only one STP on all the
VLANs and it does not provide multiple instances.
While the existing STP is a protocol to prevent a loop in a LAN domain, MSTP establishes
STP per VLAN in order to realize routing suitable to VLAN environment. It does not need
to calculate all STPs for several VLANs so that traffic overload could be reduced. By re-
ducing unnecessary overload and providing multiple transmission routes for data forward-
ing, it realizes load balancing and provides many VLANs through Instances.
8.3.3.1 MSTP
In MSTP, VLAN is classified to groups with same configuration ID. Configuration ID is
composed of revision name, region name and VLAN/instance mapping. Therefore, to
have same configuration ID, all of these tree conditions should be the same. VLAN classi-
fied with same configuration ID is called an MST region. In a region, there is only one
STP so that it is possible to reduce the number of STP comparing to PVSTP. There’s no
limitation for region in a network environment but it is possible to generate Instances up to
64. Therefore instances can be generated from 1 to 64. Spanning-tree which operates in
each region is IST (Internal Spanning-Tree). CST is applied by connecting each span-
ning-tree of region. Instance 0 means that there is not any Instance generated from
grouping VLAN, that is, it does not operate as MSTP. Therefore Instance 0 exists on all
the ports of the equipment. After starting MSTP, all the switches in CST exchange BPDU
and CST root which is decided by comparing their BPDU. Here, the switches that do not
operate with MSTP have instance 0 so that they can also join BPUD exchanges. The op-
eration of deciding CST root is CIST (Common & Internal Spanning-Tree).
Legacy 802.1d
Legacy 802.1d
CST Root & IST Root
Switch A
Switch B
Switch D
Switch C
Switch E
Instance 2
Instance 3
Instance 2
Instance 1
Region B (IST)
IST Root
CST
Region A (IST)
Fig. 8.23
CST and IST of MSTP (1)
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