With the explosion in the use of Internet, there is an obvious trend that IP-based critical applications (E-Transaction, etc.) are increasing dramatically. Therefore, it is required to build high performance routers (HPRs) with high availability (HA). Though researches on HA router hardware are fairly mature, investigations on HA routing software are still at the preliminary stage. This paper systematically presents possible implementation approaches on HA routing software and classifies them into three categories: protocol extension, state synchronization and packet duplication. This paper innovatively models and analyzes these HA routing software schemes in continuous-time Markov chains (CTMC). Based on the modeling, this paper also introduces numerical results and evaluates these different HA approaches.