Mstp Bridge And Port Roles - Siemens RUGGEDCOM ROX II User Manual

Chapter 5
Setup and Configuration
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 5.36.3.2

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
Boundary Ports
524
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.
The Master Port, which is unique in an MSTP region, is the CIST Root Port of the CIST
Regional Root, and provides the minimum cost path to the CIST Root for all MSTIs.
A Boundary Port is a port on a bridge in an MSTP region that connects to either: a bridge
belonging to a different MSTP region, or a bridge supporting only RSTP or legacy STP. A
Boundary Port blocks or forwards all VLANs from all MSTIs and the CIST alike.
A Boundary Port may be:
RUGGEDCOM ROX II
CLI User Guide
MSTP Bridge and Port Roles