Being a JTAC, following capabilities are required. 

5+ years of experience in IT industry as a technical support engineer on router & swtich devices. 

Advanced level L2, L3 protocol knowledge. 

Deep dive knowledge over routing protocols such as OSPF, ISIS, MPLS/vpn, … etc..

Advanced knowledge on HW & SW on computer architecture. 

Advanced knowledge on Unix operating systems. Programming experiences are to be preferred. 

Written & verbal English capability, enough to carry on technical discussions among engineers.