1. For a network forwarding element, a data-plane forwarding circuit comprising: a load balancer to forward message flows to different nodes of a node group, the load balancer comprising: (i) a set of storages to store a plurality of different address mapping sets with each address mapping set corresponding to a different set of nodes in the node group; (ii) a destination selector to receive a set identifier for each message flow and to select a node for the message flow from the mapping set identified by the set identifier received for the message flow; (iii) a set ID allocator to assign (1) a first set identifier for each message flow processed during a first sub-interval of an update interval when the node group is being modified, and (2) first and second set identifiers for each message flow processed during a second sub-interval of the update interval when the node group is being modified; and (iv) a cache stage that during the first sub-interval stores values that identify the message flows processed during the first sub-interval, and during the second sub-interval assigns the first set identifier to each message flow when the cache stage stores a value that identifies the message flow as being processed during the first sub-interval, and assigns the second set identifier to each message flow when the cache stages does not store such a value for the message flow.