This writable value sets the ifIndex of the physical interface that acts as the traffic source for this row's Level-1 aggregation, and must match a valid aggregator listed in the capability table. Admins write this to actually build an aggregation, since rows sharing the same value are combined into one aggregation group. Setting aggrL1AggregatorIndex to the ifIndex of if.A on multiple rows groups their aggregable interfaces into a single aggregation fed by if.A, matching the aggregation diagram in the MIB's own description.