The router scans the entire routing table, searching for an exit interface that is connected to the destination network. Note this is CPU intensive and no longer practiced because it would quickly bog down a router.
Enabled by default on all interfaces that support fast switching. To conserve resources, the router builds a table in high speed cache to prevent process switching every packet it receives.
When a router receives a packet sent to a destination not already in cache, it will be process switched.
After finding the exit interface and determining the Next Hop IP, the router will place this information in the cache.
You can enable fast switching per interface by issuing
conf t int fa0/0 ip route-cache
Review that fast switching has been enabled
Lab-C#show ip int fa0/0 | in IP fast IP fast switching is enabled IP fast switching on the same interface is disabled
You can view the table held in cache
show ip cache
You can review this process of placing entries in the cache by enabling packet debugging by issuing
debug ip packet detail
Keep in mind that when you have enabled packet debugging you can review process switching activity, however fast switched and CEF switched packets do not show up. So for the sake of troubleshooting it may be a good idea to disable fast switching (no ip route-cache) and CEF.