Spanning Tree Introduction: STP, RSTP, MSTP
©2008 Allied Telesis Inc. All rights reserved.
Software Version 5.2.1
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
18.11
Multiple Spanning Tree Instances (MSTI)
MSTP enables the grouping and mapping of VLANs to different spanning tree instances. So, an
MST Instance (MSTI) is a particular set of VLANs that are all using the same spanning tree.
In a network where all VLANs span all links of the network, judicious choice of bridge priorities
for different MSTIs can result in different switches becoming root bridges for different MSTIs.
That will result in the different MSTIs choosing different active topologies on the network. An
example of how different MSTIs can choose different active topologies on the same physical set
of links is illustrated in
Figure 18-1
.
MSTP is compatible with RSTP and STP—see
“Common and Internal Spanning Tree (CIST)”
on page 18.14
.
Figure 18-1:
Different spanning trees created by different MSTIs on the same physical layout
Physical connections
Spanning tree of MSTI= 1 containing vlans 2, 7, 9, 43
Spanning tree of MSTI= 2 containing vlans 16, 18, 24, 40
Spanning tree of MSTI= 3 containing vlans 23, 39, 50, 112
Di
ff
_MSTI_1
Root bridge for
MSTI 1
Root bridge for
MSTI 2
Root bridge for
MSTI 3
