When a new AS external link advertisement is received, it is not necessary to recalculate the entire routing table. Call the destination described by the AS external link advertisement N. N's address is obtained by masking the advertisement's Link State ID with the network/subnet mask contained in the body of the advertisement. If there is already an intra-area or inter- area route to the destination, no recalculation is necessary (internal routes take precedence).
Otherwise, the procedure in Section 16.4 will have to be performed, but only for those AS external link advertisements whose destination is N. Before this procedure is performed, the present routing table entry for N should be invalidated.