Symmetric key agreement is significant to security provisioning in sensor networks with resource limitations. A number of pairwise key predistribution protocols have been proposed, but the performance is often constrained by the unavailability of topology information before deployment and the limited storage budget within sensors. This paper proposes SBK, a self-configured scheme for boot-strapping keys in large-scale sensor networks. SBK is topology-adaptive, which requires no preloaded keying information but lets sensors compute shared keys with their neighbors after deployment. By removing the randomness inherent to key predistribution schemes, SBK achieves high connectivity (can be 100%) with small storage overhead. An improved scheme, iSBK, is also proposed to speed up the bootstrapping procedure. To the best of our knowledge, SBK and iSBK are the only pure insitu key establishment protocols that simultaneously achieve good performance in scalability, connectivity, storage overhead, and resilience.