This read-write value identifies, by lportIfIndex, which primary trunk this port is designated to back up, and by the specific convention described, 0 meaning Normal, the trunk's own ifIndex meaning it is itself the Primary, and any other trunk's ifIndex meaning this port is a Backup for that trunk, it doubles as the mechanism that determines the trunk's overall type classification. An admin sets this correctly to establish which trunk protects which, since getting the ifIndex wrong would either fail to establish the intended backup relationship or accidentally designate the wrong trunk as backup for another. For example, if trunk ifIndex 20 is meant to back up primary trunk ifIndex 12, lportPrimTrk on trunk 20 should be set to 12, while trunk 12's own lportPrimTrk would be set to its own ifIndex to mark itself as the Primary.