Technical data

BLADE OS 5.1 Application Guide
BMD00136, November 2009 Chapter 5: Spanning Tree Group
 81
Multiple Spanning Tree Group Protocol
Multiple Spanning Tree 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 STP Groups 1-32.
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
multiple Spanning-Tree instances support 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 1-32. They apply only when the CIST is used.