BGP (border gateway protocol) is widely known for some problems in terms of poor reliability, suboptimal path use, and insufficient support for load balancing because it is a single-path routing protocol. Inter-Domain multipath routing explores the underlying network AS-level path diversity to improve the Internet’s reliability, performance, and resource utilization. Thus, inter-domain multipath routing is considered a useful and necessary method to address the problems faced by BGP. This paper surveys current proposals on inter-domain multipath routing protocols and classifies these protocols into three categories: Protocols on a single announcement and multipath forwarding, protocols on multiple announcements and multipath forwarding, and new Internet routing architecture based protocols. They are compared under some different features of path diversity, control message overhead, loop-freeness property, etc. In addition to a review of existing protocols, the challenges in designing new inter-domain multipath routing protocols that could be taken as the future research direction are pointed out.