Hello Brian,
you need to build a hierarchy in routing
a)
ip route 0.0.0.0 0.0.0.0 FastEthernet0/1
ip route 0.0.0.0 0.0.0.0 FastEthernet0/0.51
this should become:
ip route 0.0.0.0 0.0.0.0 FastEthernet0/1
ip route 0.0.0.0 0.0.0.0 FastEthernet0/0.51 220
b) NAT statements
in order to be able to use Fas0/0/.51 as a backup exit path for internet we need to enable ip nat outside on it
int fas0/0.51
ip nat outside
Now, when having multiple exit points the best way is to use route-maps in NAT statement in this way we can make a match on outgoing interface
access-list 11 remark internal LAN for NAT
acccess-list 11 permit 192.168.2.0 0.0.0.255
route-map NAT1 permit 10
match ip address 11
match interface fas0/1
route-map NAT2 permit 10
match ip address 11
match interface fas0/0.51
Then you need the following statements:
ip nat source inside route-map NAT1 interface fas0/1 overload
ip nat souce inside route-map NAT2 interface fas0/0.51 overload
To complete the configuration you can use an IP SLA to track the state of the primary default route.
Hope to help
Giuseppe
