Hi, I have a T1 connected to the internet. The WAN address is assigned to the ethernet 0/0 interface. We have a secondary IP address on the ethernet 0/0 interface that is a private address that belongs to the LAN. Some before me set this up and I don't know why it was done that way. Can anyone tell me any benefits of this configuration? or reason why would you do this? I would like to get rid off the WAN addresss because it causes some minor issues. Here is the config.
no ip address
encapsulation frame-relay IETF
ip route-cache flow
no ip mroute-cache
frame-relay lmi-type ansi
interface Serial0/3/0.500 point-to-point
ip unnumbered GigabitEthernet0/0
ip access-group 112 in
ip nat outside
ip inspect vsafw out
no cdp enable
frame-relay interface-dlci 500 IETF
ip address 172.21.19.1 255.255.255.0 secondary
ip address 63.x.x.129 255.255.255.128
ip nat inside
I have a customer with a very similar configuration on their Internet facing router. I suspect that the logic of your setup is very similar to theirs. They have a point to point Frame Relay interface which is configured with ip unnumbered pointing to the LAN interface. The LAN interface has the public IP as its primary address and has a private address as secondary. The main reason that they do this is that it allows the IP block assigned to them by their provider to be used on the LAN interface. They have a number of machines on the LAN configured with public addresses. It allows the public address to be used without setting up address translation for them.
If you do show ARP on your router do you see MAC addresses on the LAN for the "public" subnet? If so then you have machines on the LAN configured with the public addresses like my customer does. If you move the public address from the LAN to the WAN as you wish to do you will need to readdress the machines on the inside and to set up translations (many of which will need to be static translations if the machines need a consistent address appearance in the Internet.