In one example embodiment, a time unconstrained fabric switch software upgrade function is described for an in-switch-software-upgrade/downgrade (ISSU) in a network switch. Prior to an ISSU, Hello protocol data unit (PDU) information is stored for relevant control protocols of associated interfaces of the switch. The Hello PDU information comprises information on messages and data exchanged by the control protocols that may be used to maintain communications links of the switch. During the configuring of the ISSU, and while the control plane of the switch is, at least partially, dysfunctional, Hello PDUs are sent on the interfaces according to the stored PDU information in order to maintain the communication links. The Hello PDU information may include a time interval value for each relevant protocol that determines when Hello PDUs are sent.