What does it mean to have fast or autonomous switching "enabled" and "disabled" on the same interface ?

Look at the above snap ^

If you enable fast or autonomous switching on an interface, packets coming from any other interface on the router are fast-switched (or autonomous-switched) to that interface. If you enable same-interface fast or autonomous switching, packets whose source and destination address are the same are fast or autonomous switched. You can use same-interface fast or autonomous switching in cases where you have Frame Relay or Asynchronous Transfer Mode (ATM) WAN links configured as subinterfaces on the same main interface. Another situation is when you are using secondary networks on LAN interfaces, as during IP address migration. In order to enable same-interface fast switching, use the ip route-cache same-interface configuration command.

Fast Switching:-

Fast switching is an improvement over process switching. The first packet of a new session is copied to the interface processor buffer. The packet is then copied to the CxBus (or other backplane technology as appropriate to the platform) and sent to the switch processor. A check is made against other switching caches (for example, silicon or autonomous) for an existing entry.

Fast switching is then used because no entries exist within the more efficient caches. The packet header is copied and sent to the route processor, where the fast-switching cache resides. Assuming that an entry exists in the cache, the packet is encapsulated for fast switching and sent back to the switch processor. Then the packet is copied to the buffer on the outgoing interface processor, and ultimately it is sent out the destination interface.

Fast switching is on by default for lower-end routers like the4000/2500 series and may be used on higher-end routers as well. It is important to note that diagnostic processes sometimes require reverting to process switching. Fast-switched packets will not traverse the route processor, which provides the method by which packets are displayed during debugging. Fast switching may also be inappropriate when bringing traffic from high-speed interfaces to slower ones—this is one area where designers must understand not only the bandwidth potential of their links, but also the actual flow of traffic.

Fast switching guarantees that packets will be processed within16 processor cycles. Unlike process-switched packets, the router’s processor will not be interrupted to facilitate forwarding.

Autonomous Switching:-

Feature on Cisco routers that provides faster packet processing by allowing the ciscoBus to switch packets independently without interrupting the system processor.

Autonomous switching is comparable to fast switching. When a packet arrives on the interface processor, it checks the switching cache closest to it— the caches that reside on other processor boards. The packet is encapsulated for autonomous switching and sent back to the interface processor. The packet header is not sent to the route processor. Autonomous switching is available only on AGS+ and Cisco7000 series routers that have high-speed controller interface cards.

If you enable fast or autonomous switching on an interface, packets coming from any other interface on the router are fast-switched (or autonomous-switched) to that interface. If you enable same-interface fast or autonomous switching, packets whose source and destination address are the same are fast or autonomous switched.

You can use same-interface fast or autonomous switching in cases where you have Frame Relay or Asynchronous Transfer Mode (ATM) WAN links configured as subinterfaces on the same main interface. Another situation is when you are using secondary networks on LAN interfaces, as during IP address migration. In order to enable same-interface fast switching, you can use the ip route-cache same-interface command.