Mstp Bridge And Port Roles - Siemens RUGGEDCOM ROS User Manual

Chapter 5
Setup and Configuration
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 5.3.3.2

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
MSTI Port Roles
148
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.
• Alternate and Backup Ports function the same as they do in
RSTP, but relative to the CIST Regional Root.
For each MSTI on a bridge:
• The Root Port provides the minimum cost path from the bridge
to the MSTI Regional Root, if the bridge itself is not the MSTI
Regional Root.
• A Designated Port provides the minimum cost path from an
attached LAN, via the bridge to the MSTI Regional Root.
• Alternate and Backup Ports function the same as they do in
RSTP, but relative to the MSTI Regional Root.
RUGGEDCOM ROS
User Guide
MSTP Bridge and Port Roles