This object holds the ifIndex of the line profile designated as the primary line in a redundancy group, with a default of 0 meaning no primary line is configured; the referenced line must be admin'd down before this field can be changed. Admins set this when building a redundant line pair, and setting it also updates the matching lineProfile entry's group assignment. For example, setting it to the ifIndex of interface 12 would designate that line as primary.