Ну вот совсем элементарный конфиг:

interface FastEthernet3/0.40
encapsulation dot1Q 40
ip address 192.168.1.50 255.255.255.0
no ip redirects
no ip unreachables
no ip proxy-arp
ip flow ingress
ip flow egress
ip nat inside
ip virtual-reassembly
no snmp trap link-status
!
interface FastEthernet3/0.100
encapsulation dot1Q 100
ip address 84.254.209.133 255.255.255.248
no ip redirects
no ip unreachables
ip flow ingress
ip flow egress
ip nat outside
ip virtual-reassembly
no snmp trap link-status
!
ip route 0.0.0.0 0.0.0.0 84.254.209.129
ip nat inside source static tcp 192.168.1.30 81 84.254.209.133 81 extendable no-alias

debug nat:
Sep  8 04:15:10.513: NAT*: o: tcp (84.254.209.100, 2526) -> (84.254.209.133, 81) [27406]    
Sep  8 04:15:10.513: NAT*: o: tcp (84.254.209.100, 2526) -> (84.254.209.133, 81) [27406]
Sep  8 04:15:10.517: NAT*: s=84.254.209.100, d=84.254.209.133->192.168.1.30 [27406]
Sep  8 04:15:13.417: NAT*: o: tcp (84.254.209.100, 2526) -> (84.254.209.133, 81) [27467]
Sep  8 04:15:13.417: NAT*: s=84.254.209.100, d=84.254.209.133->192.168.1.30 [27467]

sh ip nat tr:
Pro Inside global      Inside local       Outside local      Outside global
tcp 84.254.209.133:81  192.168.1.30:81    84.254.209.100:2526 84.254.209.100:2526
tcp 84.254.209.133:81  192.168.1.30:81    ---                ---