Analyzer/ port-mirroring Encapsulated Remote Switched Port Analyzer (ERSPAN) is a very useful addition to the analyzer family of SPAN & RSPAN where it facilitates the transport of mirrored traffic over an IP network. The traffic is encapsulated in a GRE-Header at the source router and is transferred across the network.
Note : Destination IP should be a valid unicast IP , Non local and reachable IP.
(for example: it should not be loop back address, router ip address etc)
For more details on port-mirroring on QFX
For more details on analyzer/port-mirroring on EX4300:
[Junos] GRE Configuration Example