In one embodiment, a first switch is located at a first border between first networks that utilize a STP to detect and break loops and second networks. The first switch is interconnected via the first networks with a second switch located at a second border between the first networks and the second networks. The first switch detects, via a protocol other than STP, addition of a link in the second networks that provides a new path across the second networks between the first switch and the second switch. The first switch blocks data packets from traversing through one or more network boundary ports of the first switch coupled to the first networks, while allowing STP BPDUs to traverse through the one or more network boundary ports, for a period of time sufficient for the first networks to discover the new path using STP.