User Manual Managed Switches
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STP/RSTP only depend on the physical connections, whilst MSTP configures separate Spanning
Tree instances for different VLAN groups.
The main concepts that are specific of MSTP when comparing with STP/RSTP are:
•
Multiple Spanning Tree Instances (MSTIs)
. An MST instance (MSTI) is a particular set of
VLANs that are all using the same spanning tree.
•
Regions
. An MST region is a set of interconnected switches that all have the same values for all
following MST configuration elements:
o
MST configuration name
o
Revision level
o
Mapping of which VLANs are mapped to which MST instances
Each of the MST instances created are identified by an MSTI number that identifies them only
inside the MST region. Therefore, an MSTI will never span across MST regions.
•
Common and Internal Spanning Tree (CIST)
. The CIST is the default spanning tree of MSTP,
i.e. all VLANs that are not members of particular MSTIs are members of the CIST. Also, the
spanning tree that runs between MST regions is the CIST.
The following figure shows an example of an STP/RSTP network that contains VLANs 1 and 2. The
VLANs are connected using the 802.1Q-tagged link between switch B and Switch C. By default, this
link has a port cost of 100 and is automatically blocked by STP/RSTP because the other
switch-to-switch connections have a port cost of 36 (18+18). This means that both VLANs are now
subdivided
—VLAN 1 on switches A and B cannot communicate with VLAN 1 on switch C, and VLAN
2 on switches A and C cannot communicate with VLAN 2 on switch B.
The above situation can be rectified by using MSTP. With MSTP, VLAN 1 and VLAN 2 can be
mapped to different MSTIs. Hence, each instance can have a topology independent of other
spanning tree instances.
3.6.4.3 Configuring STP / RSTP / MSTP
– Bridge Settings
The following figure indicates the STP / RSTP / MSTP parameters that can be configured. A more
detailed explanation of each parameter follows.
