Per RFC6550 188.8.131.52
3. Let L be the lowest Rank within a DODAG Version that a given node has advertised. Within the same DODAG Version, that node MUST NOT advertise an effective Rank higher than L + DAGMaxRankIncrease. INFINITE_RANK is an exception to this rule: a node MAY advertise an INFINITE_RANK within a DODAG Version without restriction. If a node's Rank were to be higher than allowed by L + DAGMaxRankIncrease, when it advertises Rank, it MUST advertise its Rank as INFINITE_RANK.