This read-create value configures the latest ATM Forum NNI/PNNI signaling specification version supported on this interface, noting that PNNI routing version is negotiated separately from ILMI, and version negotiation with the peer follows the same same-or-later logic as UNI version selection. It lets an admin control which NNI signaling protocol variant -- IISP, PNNI, E-NNI, or A-INI -- this trunk interface uses toward its neighboring switch. For example, an admin connecting two switches from different vendors over an inter-domain trunk might configure this to enni to use the ATM Forum's E-NNI variant instead of standard PNNI.