Emerging commercial live content broadcasting platforms are facing great challenges to accommodate large scale dynamic viewer populations. Existing solutions constantly suffer from balancing the cost of deploying at the edge close to the viewers and the quality of content delivery. We propose Livejack, a novel network service to allow CDN servers to seamlessly leverage ISP edge cloud resources. Livejack can elastically scale the serving capacity of CDN servers by integrating Virtual Media Functions (VMF) in the edge cloud to accommodate flash crowds for very popular contents. Livejack introduces minor application layer changes for streaming service providers and is completely transparent to end users. We have prototyped Livejack in both LAN and WAN environments. Evaluations demonstrate that Livejack can increase CDN server capacity by more than six times, and can effectively accommodate highly dynamic workloads with an improved service quality.