Mstp Bridge And Port Roles - Siemens RUGGEDCOM ROS v4.3 User Manual

Chapter 5
Setup and Configuration
The IST is also the extension inside the MST region of the CIST (see below), which spans the entire bridged
network, inside and outside of the MST region and all other RSTP and STP bridges, as well as any other MST
regions.
CST
The CST (Common Spanning Tree) spans the entire bridged network, including MST regions and any connected
STP or RSTP bridges. An MST region is seen by the CST as an individual bridge, with a single cost associated with its
traversal.
CIST
The CIST (Common and Internal Spanning Tree) is the union of the CST and the ISTs in all MST regions. The CIST
therefore spans the entire bridged network, reaching into each MST region via the latter's IST to reach every
bridge on the network.
Section 5.2.3.2

MSTP Bridge and Port Roles

MSTP supports the following bridge and port roles:
Bridge Roles
Role
CIST Root
CIST Regional Root
MSTI Regional Root
Port Roles
Each port on an MSTP bridge may have more than one CIST role depending on the number and topology of
spanning tree instances defined on the port.
Role
CIST Port Roles
160
Description
The CIST Root is the elected root bridge of the CIST (Common and
Internal Spanning Tree), which spans all connected STP and RSTP
bridges and MSTP regions.
The root bridge of the IST within an MSTP region. The CIST Regional
Root is the bridge within an MSTP region with the lowest cost path
to the CIST Root. Note that the CIST Regional Root will be at the
boundary of an MSTP region. Note also that it is possible for the CIST
Regional Root to be the CIST Root.
The root bridge for an MSTI within an MSTP region. A root bridge is
independently elected for each MSTI in an MSTP region.
Description
• The Root Port provides the minimum cost path from the bridge
to the CIST Root via the CIST Regional Root. If the bridge itself
happens to be the CIST Regional Root, the Root Port is also the
Master Port for all MSTIs, and provides the minimum cost path to a
CIST Root located outside the region.
• A Designated Port provides the minimum cost path from an
attached LAN, via the bridge to the CIST Regional Root.
RUGGEDCOM ROS
User Guide
MSTP Bridge and Port Roles