Hello Brad,
>> My desired solution, without touching router 2 or router 3, we add the router 1. ( Since router 2 and router 3 is configured by others, we want to make the least change to the system by just adding router 1. We want the network between router 2 and router 3 stays with public IP network (21.21.21.144/30).) Is this possible?
The short answer is that it is not possible to use public IPv4 subnet 21.21.21.144/30 two times on the new router R1, because this would lead to two overlapping interfaces on the same device, if you would try to configure this on a real router you would get an error message complaining of overlapping.
Also you cannot introduce the new router R1 without any changes on R2 and R3, because R3 cannot keep a public IPv4 address in 21.21.21.144/30 as this would not be supported on R1 as explained above.
As suggested by other colleagues you can / should do the following :
You can introduce new router R1, but you should use new private IP addressing between R1 and R3 and between R1 and R2.
The use of private addressing here is allowed and permits to save on public IP addresses.
On R1 you can configure appropriate routing for example static routes to reach the networks behind R2 and R3 respectively. R2 and R3 will have a default static route pointing to R1 IP address directly connected to them.
On R1 interface between R1 and R3 and interface between R1 and R2 will be NAT inside
ip nat inside in the configuration interface level.
On R1 the interface to the ISP will be NAT outside (again using ip nat outside in config interface mode).
at this point you can split the ISP pool of public IP addresses in two sub blocks one to be used for IP sources belonging to R2 behind IP subnets and one for R3 behind route subnets.
You will use each NAT pool with the overload option to allow multiple internal hosts to access internet at the same time.
So from 21.21.21.240/29 you can get two /30 subpools to use with NAT as explained above.
Hope to help
Giuseppe
