We study the state consensus problem for linear shift-invariant discrete-time homogeneous multiagent systems (MASs) over time-varying graphs. A novel approach based on the small gain theorem is proposed to design the consensus control protocols for both neutrally stable and neutrally unstable MASs. It is shown that the state consensus can be achieved for neutrally stable MASs under the uniform observability condition; for neutrally unstable MASs, the state consensus entails more than the uniform observability. Two numerical examples are worked out to illustrate our consensus results.