The process that gives rise to a rhombomere boundary. This process pertains to the initial formation of a boundary delimiting a rhombomere. Rhombomeres are transverse segments of the developing rhombencephalon that are lineage restricted, express different genes from one another, and adopt different developmental fates. Rhombomeres are numbered in anterior to posterior order. [ GOC:jid GOC:dph http://purl.obolibrary.org/obo/go/references/0000021 GOC:cls GOC:dgh ]