This value identifies the backup interface of a link backup pair and, together with hm2LinkBackupPrimaryInterface, forms the index for a row in the link backup interface table. Like its primary counterpart, it is accessible only for notification and appears in the hm2LinkBackupStatusTrap when the pair's forwarding/blocking status changes, letting an admin identify which redundant link the trap concerns. For example, a trap identifying hm2LinkBackupBackupInterface as ifIndex 8 tells the admin that interface 8, acting as the backup link, has just changed status.