Inter domain service routing is an element in the success of Next Generation Network. Service requests, such as the INVITE request in Session Initiation Protocol [21] may need to be redirected. Service Path (SPath) can be used to hold the server paths and service information. The length of SPath increases as the number of hops in a redirection increases. The overhead for service routing which uses SPath also increases. Thus it is desirable to optimize SPath to ensure efficiency and scalability of protocols involving service routing. In this paper, we propose a re-routing strategy to optimize service routing, and demonstrate how this strategy can be applied to SPath to enhance the efficiency and scalability of Service Network Graph (SNG). The formal proof for SPath optimization also forms the basis of Authentication Delegation in SNG.