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When I connect two (Zyxel) routers in a cascade configuration, I don't know how to set the right address on the Lan Interface between the outer router (the one on the top of the diagram) and the inner. In the picture below I drew two example diagrams. The first is a simple router to router connection, while in the second example, I have set the outer router in the bridge mode. What is the right Lan interface address for the outer router in the both cases?

Example

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R2 LAN IP should be in Same network as WAN Interface. In other hand, R2 LAN IP should be in same subnet as R1 WAN Interface.

In your second diagram there is no requirement for IP address because it is in bridge mode. When Router in Bridge mode it does not support to IP Routing. It may have requirement of Layer 2 configurations.

A network bridge is a layer 2 device (typically a switch), using MAC addresses with ethernet. A router is a layer 3 device (typically a router), using IP address with ethernet. So a router, in bridge mode, doesn’t route, it switches, meaning there’s no IP routing or NAT between networks, it simply extends the layer 2 domain across the device so that the “WAN” port of the router, and the “LAN” ports of the switch, are all in the same LAN with no routing (or layer 3) network separation.

Reference

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  • You have said: "R2 LAN IP in Same network as WAN Interface". My doubt is: "should be" or "have to be" ? In my tests it works whatever the address I use. while logically I think "R2 LAN IP" and "R1 WAN IP" must be on the same subnet. – Andrea Borgogelli Avveduti Jun 11 '19 at 9:28
  • The theory and practice sometimes seem not to coincide. Infact, in the second example, when I configure the router in bridge mode, it asks me for the address for the VLAN on the LAN interface. – Andrea Borgogelli Avveduti Jun 11 '19 at 9:30
  • Yes. It is must. R2 LAN IP must be in same subnet which used in R1 WAN. – infra Jun 11 '19 at 9:31
  • VLAN is a Layer 2 configuration, You need to create separate VLAN to start Communications. What is a device modal? – infra Jun 11 '19 at 9:32
  • The router is a Zyxel SBG5500-A. In bridge mode, it requires a VLAN setup, with a relative ip address for the Lan interface. – Andrea Borgogelli Avveduti Jun 11 '19 at 9:34
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In between routers, you can choose any IP subnet you like, preferrably a private IP subnet that doesn't interfere with anything else in the network.

If the bottom router's upward interface uses 10.10.10.1/24(?) you need to use any other address from 10.10.10.0/24 on the upper router's downward interface - both interfaces need to use the same subnet.

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