Should I / Can I avoid double-NAT ?

RouterOS Beginner Basics
1

Dear Forum,

I have a (planned) setting with a private LAN connected to two ISPs. Only the first WAN link (via ADSL) is currently configured.
The incoming link from the ISP is connected to an ADSL modem/router (Fritzbox 7312) and then (via Ethernet) to the Mikrotik.
config.png
I wonder if it is useful that both the modem and the Mikrotik NAT the IP traffic.
I am more concerned about the (increased) complexity, less about the performance.

(How) should I avoid the double-NAT ?

Thanks a lot for any help.

Added later:
I think the answer is simple, but please comment:

I do NOT have to NAT twice, so the RB interface that connects to FritzBox should not perform NAT. My current configuration:

[admin@MikroTik] /ip firewall nat> print   
Flags: X - disabled, I - invalid, D - dynamic 
 0   ;;; default configuration
     chain=srcnat action=masquerade out-interface=ether1-gateway

should just be disabled:

[admin@MikroTik] /ip firewall nat> disable 0

or removed:

[admin@MikroTik] /ip firewall nat> remove 0

Is this correct?


Walt

2

My quess would be that, in addition, you would have to insert a manual routing entry to the Fritzbox, in order to route all traffic to the .88.0 network via the Mikrotik. But I want to ask, have you considered using the Fritzbox only as a bridge? Then it would be transparent and the NAT would be done by MkTik.

3

I am not familiar with the fritz box, but normal ADSL modems have the possibility to switch to transparent bridge mode when it acts like media convertor. I suggest to use such mode to have full control over both wans on single mikrotik device.

4

It depends on the type of FritzBox and the software version running on it.
I’ve had 2 types and the newer was not able to be put in bridge mode and the oldest one only when NOT running latest firmware.
So in order to put the Fritzbox in bridge, check the various firmware versions.