Static NAT with two outside interfaces

Question

I have a router, which performs NAT on two outside interfaces with load balancing and had a task to allow inbound connection to be forwarded to the specific host inside on a well known port.
here is example
interface Fas0/0
ip nat outside
interface Fas0/1
ip nat outside
interface Vlan1
ip nat inside
ip nat inside source route-map rm_isp1 pool pool_isp1
ip nat inside source route-map rm_isp2 pool pool_isp2
all worked fine
then i tried to add static nat
ip nat inside source static tcp 10.0.0.1 25 interface Fas0/0 25
ip nat inside source static tcp 10.0.0.1 25 interface Fas0/1 25
and in result only last static NAT line appeared in config.
the solution was to use interface's IPs instead of names. that helped but isn't that a bug?

Answer 1

In this scenario, we are trying to access a mail server located at
10.0.0.1 from outside and we have two outside IP, let's say, 71.1.1.1 and
69.1.1.1.
With CEF Enabled
Packet comes in to Fa0/0 interface with Source IP 66.x.x.x and
Destination IP 71.1.1.1. Our NAT rule translates this to 10.0.0.1.
Packet goes to 10.0.0.1. The return packet goes to the LAN interface
first and the routing rule is determined *before* the packet is
translated.
Packet source IP at this point is 10.0.0.1 and destination is
66.x.x.x. Now, based on CEF, it will go out via Fa0/0 or Fa0/1,
irrespective of the way it came in. Because of this, with CEF enabled
this will not work. CEF is per-destination.
So, let's say somebody on outside tried to access this server using 71.1.1.1, then he would
expect a reply from 71.1.1.1 which may or may not be true as the traffic could be Nat'd to 69.1.1.1 or 71.1.1.1.
If it gets reply packet from 71.1.1.1, it should work.
If it gets it from 69.1.1.1, it will simply drop it as it never sent a
packet to 69.1.1.1.
With CEF and Fast Switching Disabled
Same steps as above, only that the packet is sent to the process level
to be routed. At this point, the packets will be sent out in a round
robin fashion. One packet will go out via the Fa0/0 and the other via the
Fa0/0. This will have a constant 50% packet loss and is also not a
viable solution.
So, what are you trying to achieve is not possible on Cisco router.
HTH,
Amit Aneja