25. Spanning Tree
ROX™ v2.2 User Guide
237
RuggedBackbone™ RX5000
25.2.2. MSTP Bridge and Port Roles
25.2.2.1. Bridge Roles:
CIST Root
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.
CIST Regional Root
The root bridge of the IST within an MST region. The CIST Regional Root is the bridge within an MST
region with the lowest cost path to the CIST Root. Note that the CIST Regional Root will be at the
boundary of an MST region. Note also that it is possible for the CIST Regional Root to be the CIST Root.
MSTI Regional Root
The root bridge for an MSTI within an MST region. A root bridge is independently elected for each MSTI
in an MST region.
25.2.2.2. Port Roles:
Each port on an MST bridge may have more than one role depending on the number and topology of
spanning tree instances defined on the port.
CIST Port Roles
• 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 (see below), 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 have the same sense that they do in RSTP, described in
, under “Roles”, but relative to the CIST Regional Root.
MSTI Port Roles
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 have the same sense that they do in RSTP, described in
, under “Roles”, but relative to the MSTI Regional Root.
The Master Port, which is unique in an MST region, is the CIST Root Port of the CIST Regional Root,
and provides the minimum cost path to the CIST Root for all MSTIs.
Boundary Ports
A Boundary Port is a port on a bridge in an MST region that connects to either of: 1) a bridge belonging
to a different MST region, or 2) 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:
• The CIST Root Port of the CIST Regional Root (and therefore also the MSTI Master Port).