If Router receives fragmented traffic what it will do? Who will assemble the fragmented packet?

If router receives a protocol data unit (PDU) larger than the next hop’s MTU, it has two options if the transport is IPv4: drop the PDU and send an Internet Control Message Protocol (ICMP) message which indicates the condition Packet too Big, or fragment the IP packet and send it over the link with a smaller MTU.

The host which receives a fragmented IP packet, it has to reassemble the datagram and pass it to the higher protocol layer. Reassembly is intended to happen in the receiving host but in practice it may be done by an intermediate device in this case router,eg network address translation (NAT) may need to re-assemble fragments in order to translate data traffic