For horizontal links and uplinks, this attribute contains the port identifier of the port at the remote end of the link as assigned by the remote node. If unknown, the PNNI protocol entity sets this attribute's value to zero. For nodes, this attribute contains the port identifier of the port at the other end of the spoke or bypass from the originating port. When the originating port ID is zero, a value of zero indicates the default radius. When the originating port ID is non-zero, a value of zero indicates the nodal nucleus.