What I would like to do is basically bridge 2 eth ports together or set up in a VLAN to act as the WAN side of the router. I do not want to set up a 2nd Internet connection to do any failover or load balancing. This would be for a situation where a cable modem has a single eth port but mulitple static public IPs. I would want to plug my ER into the cable modem for an isolated VoIP network, then plug the customer's existing router into the "bridged" ER eth port for their normal Internet access. This is all to avoid putting yet another switch before the firewall that could fail.
I'm fairly new to ERs so I'm not quite sure if this was even possible, how easliy is is done, and whatever potential flaws could be there. Any guidance would be greatly appreciated.
Connection scenario:
WAN Side
Cable modem -----> EReth0 (WAN IP1)
EReth1 ----------------------> CPE WAN (WAN IP2)
-------------------------------------------------------------------------------------------------------------
LAN Side EReth2--| |
| CPE LAN (192.168.1.x)
| --------------------------------------------------------------
|-------> VoIP LAN (10.10.10.x)