"All routers are on process switching but when i am putting new 4331 its showing fast switching thats'y i want to disabled fast switching"
Interesting, because, we generally try to obtain/use the "fastest" routing path, and process switching is considered (I recall) about the worst. I.e. we try to avoid, intentionally, using process-switching.
Ideally, any "switching path" will forward packets to their destination; usually the only difference between them, is how efficiently they do this, again, "faster" methods are generally considered "better" methods.
One major difference, "unique" to process-switching, I recall (?) it does packet-by-packet to the destination, rather than flow-by-flow to the destination. This doesn't matter on a single path, but is a major difference if multiple paths are being used. (Here too, without discussing "why", packet-by-packet is considered a [much] worst choice vs. flow-by-flow. BTW, also, sometimes the latest/best switching-path, CEF, supports packet-by-packet.)
So far, it seems the logic of not using MC distributed, or doing unicast process-switching, is because that's how your 2851 is configured. Is that the only reason? If so, normally using better, more efficient, forwarding options is recommended. I.e. try using 4331's defaults, and see if you continue to get the results you need.
That said, sometimes you can force newer devices to use older and less efficient forwarding methods. But, also sometimes, Cisco has "determined", no one should be doing/using such old approaches, and they no longer support it. And, sometimes, Cisco allows a newer method to provide almost the same physical results, like CEF packet-by-packet, even when they recommend against using such an option.
