Boundary ports are bridge ports that are only connected to other MSTP (Multiple Spanning Tree Protocol) regions or 802.1D or 802.1w bridges.
The ports that are not at a region boundary are called internal ports. The boundary ports exchange only CIST BPDUs. A CIST BPDU originated from the CIST root enters a region through the CIST root port and egresses through boundary ports. This behavior simulates a region similar to an 802.1w bridge, which receives BPDUs on its root ports and forwards updated BPDUs on designated ports.
Sample MSTP Topology with Two MSTP Regions shows an MSTP network that consists of two MSTP regions. Each region has its own CIST regional root and is connected to the CIST root through master ports. The CIST regional roots in each region are the MSTP bridges having the lowest CIST external root path cost. The CIST root is the bridge with the lowest bridge ID and is an 802.1w bridge outside of either MSTP region.
MSTP Region 1 and MSTP Region 2 are connected to the CIST root through directly connected ports, identified as master ports. The bridge with ID 100 connects to the CIST root through Region 1, Region 2, or segment B. For this bridge, either Region 1 or Region 2 can be the designated region or segment B can be the designated segment. The CIST BPDUs egressing from the boundary ports carry the CIST regional root as the designated bridge. This positions the entire MSTP region as one virtual bridge.
The CIST controls the port roles and the state of the boundary ports. A master port is always forwarding for all CIST and MSTI (Multiple Spanning Tree Instances) VLANs. If the CIST sets a boundary port to the discarding state, the CIST blocks traffic for all VLANs mapped to it and the MSTIs within that region. Each MSTI blocks traffic for their member VLANs and puts their internal ports into the forwarding or blocking state depending on the MSTI port roles.
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