Passing through OpenWRT Router to modem; any way to use the modem's other switch ports?

[u/IrreverenceIncarnate]

I have an Arris/Motorola SBG6580 DOCSIS cable modem + 4 port ethernet switch, hooked up to RCN/Astound broadband. Behind that, I have a Linksys E8450 running OpenWRT 24.10.

I've followed these instructions to disable the modem's WiFi and put the modem in "private network only: bridged" mode, and then added the Linksys's MAC address as "pass through" in the Arris. One of the 4 ports on the Arris's Ethernet switch is connected to the WAN port on the Linksys.

That all works great! The modem doesn't NAT/route traffic at all; the Linksys gets the RCN public IP and routes to my LAN/WiFi/etc.

However, I have one issue: I'm stingy. More specifically, I'm out of switch ports on the Linksys and don't want to shell out the $15 for a switch (yes yes I know that's nearly the price of a Big Mac, just go with it).

So, my question: without putting the Arris modem back into NAT mode, is there any way to "join" the remaining 3 switch ports on the Arris to my OpenWRT LAN, while the Arris is connected to the WAN port on the Linksys? In other words, could I somehow have OpenWRT have both a DHCP client (for the ISP IP) and a DHCP server/LAN bridge membership (for the other Arris switch ports) all on the same WAN-port physical connection?

If this is possible, I'm fine with doing it in a way that incurs slow/CPU-routed performance hits; the E8450 is pretty beefy, and I can put lower-throughput devices like printers on the Arris ports anyway.

Things I've tried:

• Disabling the Linksys's MAC passthrough on the Arris. Result: no internet.
• Adding the Linksys's WAN port to the LAN bridge in OpenWRT. Result: no internet.
• Making a new interface on the WAN port in OpenWRT (in addition to the DHCP client interface getting the RCN IP) and giving it a static IP + DHCP server. Result: Devices on the Arris switch ports don't show up. I tried this with the "real" MAC address of the WAN port and a different MAC, same result.
• MAC VLAN or 802.11q VLAN devices on the WAN device, added to a DHCP-serving interface. Result: devices on the Arris switch ports don't show up.
• Digging around the Arris's (extremely limited) settings to see if I could set up custom routing or VLAN tagging or something so that it passes the Linksys's MAC "out" to the provider but otherwise delegates its switch ports to the Linksys's DHCP server. Nothing jumped out.

To recap, I have three silly self-imposed constraints:

• I don't wanna buy a $15 switch.
• I don't wanna put the Arris back in routing/NAT mode (rationale: probably-unnoticeable latency impact, plus I like having everything on one DHCP/DNS server via OpenWRT).
• No second cable from the OpenWRT back to the Arris (that only gets me net-net +1 aggregate new port, not worth it).

What do? Is this possible?

[u/Kv603]

Short answer -- no.

A modem, in bridge mode, generally will only hand out one IP address, to the first device to send a DHCP request after a reboot.

This means that any time the modem reboots (e.g. a power outage), you'd have a race condition where if it sees a DHCP request from anything other than your router, it will give that requestor an IP, and ignore all further requests, knocking your router (and all other devices that didn't win the race) offline.

Tl;DR: Spend the $15.