What exactly is router cascade option doing? [duplicate]

I'm trying to understand what exactly happens when routers are cascaded. All description I can find state that it is done when I want to connect router to router but I can do it without any setup.

So what is network cascade in terms of DHCP/NAT (Masqarade)/IPv6-RA for LAN-LAN and LAN-WAN?

Solution 1:

"Router cascade" is not a technical term and doesn't have a well-defined meaning in network configuration; it is just how people describe the action of connecting two routers together. (Specifically, two home gateways.) However, this is what it usually means:

• In most cases, what it is is literally just two routers connected together ("LAN-WAN" i.e. router 1's LAN → router 2's WAN) without them having any awareness of each other. No configuration involved – you unbox it, plug it in and "if it works, it works".

  This typically results in both routers thinking they're connected directly to Internet – therefore both have their own separate subnets, both do routing with NAT, both have separate "port-forwarding" configurations, you end up with two layers of everything. It indeed works but is far from optimal.

  (IPv6 is practically never a consideration when "router cascade" is done, but most likely 2nd router would attempt DHCPv6-PD from the 1st one, like it would from an ISP, and if the 1st router supports sub-delegation i.e. serving DHCPv6-PD to LAN devices, then they'd again end up with separate IPv6 subnets.)

• The other common configuration ("LAN-LAN", i.e. router 1's LAN → router 2's LAN) involves reconfiguring the 2nd router to act as a pure bridge. Home gateways already perform bridging between all of their LAN and Wi-Fi ports as part of their standard functionality, so this really means just disabling the features that make it a router.

  Specifically, the physical LAN-LAN connection (i.e. avoiding the WAN port) will bypass the 2nd router's routing, NAT, and firewalling, so what's left is disabling the DHCP and UPnP-IGD services on the 2nd router (preventing it from advertising itself as the default gateway).

  In this mode, as the 2nd "router" is only working as a bridge, it is invisible as far as both IPv4 DHCP and IPv6 RA/DHCPv6 are concerned: there's just one flat subnet that spans across both devices, and all devices connected to the 2nd "router" will directly receive RAs and DHCP offers from the 1st one.

The third option (keeping routing and two separate subnets, but disabling NAT/firewalling and setting up static routes between the two LAN subnets) is relatively uncommon as far as the "router cascade" terminology goes, mainly because a lot of home gateways just don't offer route configuration as a feature (and/or don't allow disabling NAT either).

The IETF "Homenet" group attempts to make home gateways smarter, so that they'd recognize a "cascade" or "chain" of multiple home gateways and automatically configure routing between themselves (avoiding NAT within the network) – but so far, I don't think I've seen any home routers with that feature deployed. (I think the 'hncpd' software is available on DD-WRT though, but that's pretty much it?)

Note: Some manufacturers also have their proprietary features that detect router cascading and automatically configure all but one routers as pure bridges (for example, ASUS has "AiMesh" and Huawei has… I don't remember what it's called, but it's there). This only works if all involved routers are from the same manufacturer and have the automatic detection feature.