This chapter covered many topics outside of CEF and presented a model that eventually leads to CEF troubleshooting. CEF is a scapegoat for many IP connectivity issues. The most common IP connectivity issues that are not CEF issues occur at the physical layer and Layer 2, such as spanning-tree or HSRP failures. Therefore, you should troubleshoot based on the OSI model. If a device uses hardware switching, conduct additional troubleshooting to isolate the issue.