Nat – How to make IPSEC over double NAT

Well, You can extend the LANs with L2TPv3 and protect the setup with IPsec.

Here's example configuration: https://www.softether.org/index.php?title=4-docs/2-howto/L2TP//IPsec_Setup_Guide_for_SoftEther_VPN_Server/Cisco_IOS_L2TPv3//IPsec_Edge-VPN_Router_Setup

And here's another one, modern, using FlexVPN - unfortunately, depending on your IOS version, you may not be able to use it: http://www.cisco.com/c/en/us/support/docs/security/flexvpn/116207-configure-l2tpv3-00.html

I did some testing for that kind of setups, bear in mind transporting all the L2 traffic over IP will cost you bandwidth. If you need any help in setup/troubleshooting, let me know.

As for more obvious problem - connecting 891's to ISPs - just point default network to them as gateway. From this setup the routing itself is typical in terms of configuration.

To test your scenario I set up the following lab:

The 10.0.0.0/24 network is your -RangeOfIPs-

When traffic comes from 10.0.0.0/24 it will be NATed to 192.168.0.21. Traffic sourcing from 192.168.0.114 will be NATed to 10.1.1.21.

Configuration:

R3(config)#int f0/0
R3(config-if)#ip nat outside
R3(config-if)#int f0/1
R3(config-if)#ip nat inside

The above commands define the interfaces as outside and inside.

R3(config)#ip nat inside source static 192.168.0.114 10.1.1.21

This command translates the inside local address of 192.168.0.114 to an inside global address of 10.1.1.21.

R3(config)#access-list 1 permit 10.0.0.0 0.0.0.255

This access-list will define which hosts on the outside that will get NATed.

R3(config)#ip nat pool NAT_POOL 192.168.0.21 192.168.0.21 netmask 255.255.255.0

We create a NAT pool consisting of a single address.

R3(config)#ip nat outside source list 1 pool NAT_POOL add-route 

Then we configure so that hosts matching access-list 1 will get NATed to 192.168.0.21.

It is important to configure add-route here or to add a static route because when doing inside to outside NAT, NAT takes place before routing in the order of operations. That means that R3 must have a route for 10.1.1.21.

R3 now has the following NAT table:

R3#show ip nat translations 
Pro Inside global      Inside local       Outside local      Outside global
--- ---                ---                192.168.0.21       10.0.0.1
--- 10.1.1.21          192.168.0.114      ---                ---

Note that R4 has configured with an IP and ip routing turned off to emulate a host. Debugging of ICMP on R1 is enabled and debugging of ip nat on R3 is also enabled.

R1#debug ip icmp
ICMP packet debugging is on

R3#debug ip nat 
IP NAT debugging is on

A ping is then issued from R1:

R1#ping 10.1.1.21 so f0/1

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.21, timeout is 2 seconds:
Packet sent with a source address of 10.0.0.1 
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 80/86/104 ms
R1#
ICMP: echo reply rcvd, src 10.1.1.21, dst 10.0.0.1
ICMP: echo reply rcvd, src 10.1.1.21, dst 10.0.0.1
ICMP: echo reply rcvd, src 10.1.1.21, dst 10.0.0.1
ICMP: echo reply rcvd, src 10.1.1.21, dst 10.0.0.1
ICMP: echo reply rcvd, src 10.1.1.21, dst 10.0.0.1

Debug and NAT table from R3:

NAT*: s=10.0.0.1->192.168.0.21, d=10.1.1.21 [15]
NAT*: s=192.168.0.21, d=10.1.1.21->192.168.0.114 [15
NAT: s=192.168.0.114->10.1.1.21, d=192.168.0.21 [15]
NAT: s=10.1.1.21, d=192.168.0.21->10.0.0.1 [15]

R3#show ip nat translations
Pro Inside global      Inside local       Outside local      Outside global
--- ---                ---                192.168.0.21       10.0.0.1
icmp 10.1.1.21:3       192.168.0.114:3    192.168.0.21:3     10.0.0.1:3
--- 10.1.1.21          192.168.0.114      ---                ---

I think that this is the kind of configuration you are looking for.

However, note that there is a caveat because there is no overload (PAT) available for outside to inside translation. That means that as soon as one of your hosts communicate with 192.168.0.114, there will be no free IP's in the pool. What you can do is to increase the pool size so that you reserve maybe 10 IP's that are only used for NAT.