Security modifications to legacy network protocols are expensive and disruptive. This paper outlines an approach, based on external security monitors, for securing legacy protocols by deploying additional hosts that locally monitor the inputs and outputs of each host executing the protocol, check the behavior of the host against a safety specification, and communicate using an overlay to alert other hosts about invalid behavior and to initiate remedial actions. Trusted computing hardware provides the basis for trust in external security monitors. This paper applies this approach to secure the Border Gateway Protocol, yielding an external security monitor called N-BGP. N-BGP can accurately monitor a BGP router using commodity trusted computing hardware. Deploying N-BGP at a random 10% of BGP routers is sufficient to guarantee the security of 80% of Internet routes where both endpoints are monitored by N-BGP. Overall, external security monitors secure the routing infrastructure using trusted computing hardware and construct a security plane for BGP without having to modify the large base of installed routers and servers.