When you use the NAT router feature on a Catalyst 6500 with Supervisor Engine 1/2 and MSFC/MSFC2, packets that traverse the NAT outside interface can (in certain configurations) undergo software routing instead of Layer 3 (L3) switching. The software routing can occur regardless of whether the packets require translation. For packets that traverse the NAT outside interface, the redirection to MSFC for software routing should occur for only those packets that require NAT. Cisco IOS Software only translates traffic that traverses from NAT inside interfaces to NAT outside interfaces. Create the access control list (ACL) for use with NAT to be more specific. Have the ACL limit the software-handled packets to only those packets that require NAT translation. For example, if you use a general ACL, such as permit ip any any, to specify the traffic that requires NAT, all traffic inbound or outbound on the NAT outside interface is software routed. Traffic that does not originate in the NAT inside interfaces or have the NAT inside interfaces as a destination is also software routed. If you use a more specific ACL, such as permit ip 192.168.1.0 0.0.0.255 any, only the NAT outside traffic that matches the ACL is software routed.
