How does this LAN design work?

Question

Computer networks

Good evening experts! Please help in understanding the essence, namely, I don’t catch up, how does this network scheme work?
I will describe the scheme itself:
Several dedicated ip addresses from the provider come to the switch. Accordingly, from ports 1, 2 and 3 they go to routers. Router A is connected to a dumb switch to which office computers are connected.

However, from router B and C, the lan also goes to this switch, and if you connect to them directly, to their lan ports, then I get the ip address from router A (it is also the gateway) and its external IP address! But, the most interesting thing is that if I turn off router B, then the Internet disappears on router D and the equipment behind it, and router D itself has a different external ip address. Router C works in the same way.

Now I have a question, how is the external ip address transmitted through a router, which in its behavior resembles an access point, then through a switch and to another router!
Maybe everything is simpler than a steamed turnip, but I have never encountered this, please explain.

Thanks in advance!

Answer 1

I explain, to understand the picture:
White IPs 1.1, 1.2, 1.3 come to the first switch.
Router A has an external address of 1.1 - internal subnet A.0/24, the gateway address itself is A.1.
Router B (when I connect to its LAN) gives me subnet A.0/24, the external address is similar to router A - 1.1, the gateway address is A.1 (it turns out that there is no NATA).
Router C (when I connect to its LAN) gives me subnet A.0/24, the external address is similar to router A - 1.1, the gateway address is A.1 (it turns out that there is also no NATA).
Router D (when I connect to its LAN) gives me the D.0/24 subnet, the external address is 1.2 (everything is standard here, most likely the static in the VAN is broken). This router is connected to router B because when it is turned off, the Internet disappears - in general.
Logically, there is a router somewhere that is connected to router C, but when it was turned off, no one "ran" so there is no information here.
UPD, I think I know how this miracle works! In general, the main router A acts as a gateway for one network + DHCP, the other two routers B and C work as a non-DHCP gateway in the same subnet as router A. In router D, in turn, one of the routers B or C, which allows it to go outside with another white IP.

Answer 2

For a more complete understanding of what is happening, indicate the IP addresses on the diagram (specify where they are set statically, where they are obtained using DHCP) and vlan (if used), perhaps it will be clearer. Do I understand correctly that each of the routers A, B, C, D has NAT configured between the WAN and LAN interfaces?

How does this LAN design work?

For now, I assume that it’s like in a student dormitory the next morning after a party - whoever got up earlier (answered with a DHCP message), that’s the sneaker (the one who NATs user traffic).

However, from router B and C, the lan also goes to this switch, and if you connect to them directly, to their lan ports, then I get the ip address from router A (it is also the gateway) and its external IP address!

You are most likely getting the IP address and default route from Router A.

But, the most interesting thing is that if I turn off router B, then on router D and the equipment behind it, the Internet disappears, and router D itself has a different external ip address.

Router D most likely got the IP address for the WAN interface and the default route from Router B. When it is turned off, it gets the IP address and route from the other (A,C) router.

internet goes down

Temporarily or completely?
It is not clear what problem the person who created this was solving . If there are no more connections to routers A,B,C, their functionality can be combined in one device.

Answer 3

Did you set up priorities (metrics) and routing paths? Otherwise, D considers router A to be the only gateway in your campaign and, in case of loss, does not switch to a backup channel / route.

Answer 4

tin and not topology