As Software-Defined Networking has gained increasing prominence, new attacks have been demonstrated which can corrupt the SDN controller's view of network topology. These topology poisoning attacks, most notably host-location hijacking and link fabrication attacks, enable adversaries to impersonate end-hosts or inter-switch links in order to monitor, corrupt, or drop network flows. In response, defenses have been developed to detect such attacks and raise an alert. In this paper, we analyze two such defenses, TopoGuard and Sphinx, and present two new attacks, Port Probing and Port Amnesia, that can successfully bypass them. We then develop and present extensions to TopoGuard to make it resilient to such attacks.