Boundary Ports In the Cisco prestandard implementation, a boundary port connects an MST region to a single spanning-tree region running RSTP, to a single spanning-tree region running PVST+ or rapid PVST+, or to another MST region with a different MST configuration. A boundary port also connects to a LAN, the designated device of which is either a single spanning-tree switch or a switch with a different MST configuration. There is no definition of a boundary port in the IEEE 802.1s standard. The IEEE 802.1Q-2002 standard identifies two kinds of messages that a port can receive:
• internal (coming from the same region) • external (coming from another region) When a message is internal, the CIST part is received by the CIST, and each MST instance receives its respective M-record. When a message is external, it is received only by the CIST. If the CIST role is root or alternate, or if the external BPDU is a topology change, it could have an impact on the MST instances. An MST region includes both devices and LANs. A segment belongs to the region of its designated port. Therefore, a port in a different region than the designated port for a segment is a boundary port.
This definition allows two ports internal to a region to share a segment with a port belonging to a different region, creating the possibility of a port receiving both internal and external messages. The primary change from the Cisco prestandard implementation is that a designated port is not defined as boundary, unless it is running in an STP-compatible mode. If there is a legacy STP device on the segment, messages are always considered external. Note The other change from the Cisco prestandard implementation is that the CIST regional root device ID field is now inserted where an RSTP or legacy IEEE 802.1Q device has the sender device ID.
The whole region performs like a single virtual device by sending a consistent sender device ID to neighboring devices. In this example, Switch C would receive a BPDU with the same consistent sender device ID of root, whether or not A or B is designated for the segment. IEEE 802.1s Implementation The Cisco implementation of the IEEE MST standard includes features required to meet the standard, as well as some of the desirable prestandard functionality that is not yet incorporated into the published standard. Port Role Naming Change The boundary role is no longer in the final MST standard, but this boundary concept is maintained in Cisco’s implementation.
However, an MST instance port at a boundary of the region might not follow the state of the corresponding CIST port. Two boundary roles currently exist: • The boundary port is the root port of the CIST regional root—When the CIST instance port is proposed and is in sync, it can send back an agreement and move to the forwarding state only after all the corresponding MSTI ports are in sync (and thus forwarding). The MSTI ports now have a special primary role. Configuring Multiple Spanning-Tree Protocol Boundary Ports Configuring Multiple Spanning-Tree Protocol 7