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Configuring Spanning Tree Parameters
What is the Common Spanning Tree
The Common Spanning Tree (CST) is the overall network Spanning Tree topology resulting from STP,
RSTP, and/or MSTP calculations to provide a single data path through the network. CST provides
connectivity between MST regions and other MST regions and/or Single Spanning Tree (SST) switches.
For example, in the above diagram, CST calculations detected a network loop created by the connections
between Switch D, Switch E, and the MST Region. As a result, one of the paths was blocked.
What is the Internal Spanning Tree (IST) Instance
The IST instance determines and maintains the CST topology between MST switches that belong to the
same MST region. In other words, the IST is simply a CST that only applies to MST Region switches
while at the same time representing the region as a single Spanning Tree bridge to the network CST.
As shown in the above diagram, the redundant path between Switch B and Switch C is blocked and the
path between Switch A and Switch C is blocked. These blocking decisions were based on IST
computations within the MST region. IST sends and receives BPDU to/from the network CST. MSTI
within the region do not communicate with the network CST. As a result, the CST only sees the IST
BPDU and treats the MST region as a single Spanning Tree bridge.
What is the Common and Internal Spanning Tree Instance
The Common and Internal Spanning Tree (CIST) instance is the Spanning Tree calculated by the MST
region IST and the network CST. The CIST is represented by the single Spanning Tree flat mode instance
that is available on all switches. By default, all VLANs are associated to the CIST until they are mapped
to an MSTI.
When using STP (802.1D) or RSTP (802.1w). When using MSTP, the CIST is also known as instance 0 or
MSTI 0.
Note. When MSTP is the active flat mode protocol, explicit Spanning Tree bridge commands are required
to configure parameter values. Implicit commands are for configuring parameters when the STP or RSTP
protocols are in use. See
information.
MST Configuration Overview
The following general steps are required to set up a Multiple Spanning Tree (MST) configuration:
•
Select the flat Spanning Tree mode – Each switch runs in the default mode. MSTP is only supported
on a flat mode switch. See
•
Select the MSTP protocol – Each switch uses the default protocol. Selecting MSTP activates the
Multiple Spanning Tree. See
•
Configure an MST region name and revision level – Switches that share the same MST region
name, revision level, and VLAN to Multiple Spanning Tree Instance (MSTI) mapping belong to the
same MST region. See
•
Configure MSTIs – Every switch has a default Common and Internal Spanning Tree (CIST) instance
0, which is also referred to as MSTI 0. Configuration of additional MSTI is required to segment switch
OmniSwitch AOS Release 7 Network Configuration Guide
"Using Spanning Tree Configuration Commands" on page 5-25
"Spanning Tree Operating Modes" on page 5-20
"How MSTP Works" on page 5-12
"What is a Multiple Spanning Tree Region" on page 5-16
for more information.
March 2011
MST General Overview
for more
for more information.
for more information.
page 5-17
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