Multiple Spanning Tree Protocol 39
Chapter 3: Spanning Tree Protocol
Multiple Spanning Tree Protocol
Multiple Spanning Tree Protocol (MSTP) extends Rapid Spanning Tree Protocol
through multiple Spanning Tree Groups, using multiple VLANs in each STG. MSTP
supports up to 32 Spanning Tree instances, that correspond to Spanning Tree
Groups 1 through 32. For more information about Spanning Tree Protocol, see
“Spanning Tree Protocol” on page 31.
In Multiple Spanning Tree Protocol (MSTP), several VLANs can be mapped to each
Spanning Tree instance. Each Spanning Tree instance is independent of other
instances. MSTP allows frames assigned to different VLANs to follow separate
paths, each path based on an independent Spanning Tree instance. This approach
provides multiple forwarding paths for data traffic, enabling load balancing, and
reducing the number of Spanning Tree instances required to support a large
number of VLANs.
MSTP Region
A group of interconnected bridges that share the same attributes is called an MST
region. Each bridge within the region must share the following attributes:
Alphanumeric name
Revision number
VLAN-to-STG mapping scheme
MSTP provides rapid re-configuration, scalability, and control due to the support of
regions, and support for multiple Spanning Tree instances within each region.
Common Internal Spanning Tree
The Common Internal Spanning Tree (CIST) provides a common form of Spanning
Tree Protocol, with one Spanning Tree instance that can be used throughout the
MSTP region. CIST allows the switch to interoperate with legacy equipment,
including devices that run IEEE 802.1D (1998).
CIST allows the MSTP region to act as a virtual bridge to other bridges outside of the
region, and provides a single Spanning Tree instance to interact with them.
CIST port configuration includes Hello time, path-cost, and interface priority. These
parameters do not affect Spanning Tree Groups 1 through 32. They apply only
when the CIST is used.
MSTP Configuration Guidelines
This section provides important information about configuring Multiple Spanning
Tree Groups:
When MSTP is turned on, the switch automatically moves all VLANs to the CIST.
When MSTP is turned off, the switch moves all VLANs from the CIST to STG 1.
When enabling MSTP, you must configure a Region Name, and a default
version number of 0 (zero) is configured automatically. Each bridge in the
region must have the same name, version number, and VLAN mapping.
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