We consider several basic consensus problems in multi-agents systems with switching interconnection graphs, where the connections between any two agents are assumed bidirectional for notation simplicity. When the switching interconnection graphs are always connected, it has been shown in the literature that the Laplacian based algorithm always achieves the average consensus. We first extend the discussion to the case where disconnected interconnection graphs are involved, by showing that the average consensus is still achieved if the dwell time ratio between connected graphs and disconnected ones satisfies a specified condition. Next, we consider the case where there is no connected graph but the combination of a set of graphs is connected, and propose two switching strategies for achieving the average consensus. Several numerical examples are provided to show the algorithms.