The entry index keys is trap source name and a filter ID. The filter matches an index OID if the index_filter matches the OID of the index taking index_mask into account. Each bit of index_mask corresponds to a sub-identifier of index_filter, with the most significant bit of the i-th octet of this octet string value (extended if necessary) corresponding to the (8*i - 7)-th sub-identifier. Each bit of index_mask specifies whether or not the corresponding sub-identifiers must match when determining if an OID matches; a '1' indicates that an exact match must occur; a '0' indicates 'wild card'. If the value of index_mask is M bits long and there are more than M sub-identifiers in index_filter, then the bit mask is extended with 1's to be the required length.