Ethernet has spread its territory into metro and backbone networks. Since spanning tree protocol (STP) is impractical for backbone networks routing, many spanning tree based Layer 2 routing schemes have been proposed, such as multiple spanning tree protocol (MSTP) and MSTP based shortest path bridging (MSTP-SPB). We have developed three analytical models for evaluating these routing schemes, but they failed to analyze Intermediate System to Intermediate System protocol based shortest path bridging (ISIS-SPB), which forwards data from source to destination by multiple shortest path trees. This paper improves previous models by redefining the routing scheme variables, and presents an analytical model for evaluating Layer 2 routing schemes even for multi-path routing. Numerical results from this analytical model verify that the ISIS-SPB is much better than MSTP-SPB and STP.