The ad-hoc network [1] integrates the computer network and mobile communication without control centers, and is self-built, self-organized, and self-managed. It is very useful in military and commercial applications such as national defense readiness, emergency rescue, exploration missions, and sensor networks, all of which have a need for ubiquitous communication services without the presence of a fixed infrastructure. Route protocol design is a more difficult task in ad-hoc networks because of the frequent node mobility, limited battery energy and bandwidth, and high bit error rate. In order to manage frequent topology changes caused by node mobility and optimize multiple quality-of-service (QoS) parameters in ad-hoc networks, the control overheads must be reduced as much as possible. The alternate path route (APR) protocols improve the robustness of existing on-demand routing protocols against high node mobility through alternate path construction, thus reducing the flooding frequency and overheads of route rediscovery, whereas most existing alternate path routing protocols focus on the alternate path construction and handover and do not consider the aging mechanism of alternate paths. Therefore, it is difficult to assess the status of the APR set. Although most current APR protocols achieve as many alternate paths as possible for a single source-destination pair during route discovery at a low cost, the alternate path route set is unidirectionally constructed just from source to the destination by flooding, which is inefficient for bilateral communication. Besides, few existing APR protocols support QoS. In addition, centralized routing mechanism in ad-hoc networks with frequent topology changes may result in the increase of overheads, low reliability, and scalability, resulting in a bottleneck of traffic flow at the control center. Owing to its good distributed control mechanism, ant-colony optimization based routing protocols provide many redundant routes and reliable connectivity, which makes the system very robust against Abstract In order to periodically reassess the status of the alternate path route (APR) set and to improve the efficiency of alternate path construction existing in most current alternate path routing protocols, we present a cross-layer design and ant-colony optimization based load-balancing routing protocol for ad-hoc networks (CALRA) in this paper. In CALRA, the APR set maintained in nodes is aged and reassessed by the inherent mechanism of pheromone evaporation of ant-colony optimization algorithm, and load balance of network is achieved by ant-colony optimization combining with cross-layer synthetic optimization. The efficiency of APR set construction is improved by bidirectional and hop-by-hop routing update during routing discovery and routing maintenance process. Moreover, ants in CALRA deposit simulated pheromones as a function of multiple parameters corresponding to the information collected by each layer of each node visited, such as the distance from their source node, the con...