BGP is a core Internet routing protocol. The Internet inter-domain routing relies on the exchange of BGP routing information. BGP has significant vulnerabilities, which have been found to cause problems such as prefix hijacking, route leak and Internet-targeted denial of service attack. First, by analyzing BGP route propagation and BGP routing policies, the fundamental flaw in the design of the protocol is revealed. The paper then discusses possible threats to BGP and presents a route leak model, which contributes to the description of its characteristics. Second, the existing defense mechanisms for BGP security are generalized, and their shortcomings are exposed. The paper then classifies various BGP security-enhancing technologies and studies them in detail to explore their advantages and disadvantages. Finally, the research trends of BGP security are discussed in this paper.