1. A computer-implemented method, comprising: instantiating, by one or more computing systems of a configurable network service, a virtual computer network for a client based on configuration information received from the client, wherein the configuration information indicates that a virtual router device of the virtual computer network interconnects multiple computing nodes of the virtual computer network; intercepting, by the one or more computing systems, a routing communication that is directed to the virtual router device and includes routing information for the virtual computer network, wherein the routing information indicates that a first computing node of the multiple computing nodes is using a first virtual network address of the virtual computer network in a different manner than was previously specified in the configuration information; updating, by the one or more computing systems and based on the routing information, the configuration information for the virtual computer network to associate the first virtual network address with the first computing node; and sending, by the one or more computing systems and based at least in part on the updated configuration information, a communication directed to the first virtual network address to the first computing node, wherein the sending of the communication includes emulating functionality of the virtual router device.