I assume you can use a simple setup …
- you have one ISP ethernet cable (this will support the PPPoE connections)
- you have 2 independent PPPoe client interfaces, one each for your ISP PPPoE accounts
- This is your WAN side, with 2 interfaces. WAN and LAN normally are not bridged. So this ethernet interface, nor the 2 PPPoe interfaces are connected to a bridge
The traversal from LAN to WAN is with the default NAT rules of the firewall. You will need 2 NAT rules instead of one. You will also need 2 default routes.
- If you need a bridge or VLAN for the LAN side of the network, depends on the design there.
Many possibilities for the LAN side. It can perfectly be done with just 2 ethernet interfaces , or 2 bridges with some ethernet interfaces, or one bridge with or without 2 VLAN
-Take 2 LAN interfaces …
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one IP address per interface (192.168.3.1 and 192.168.4.1)
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one DHCP server per ethernet interface (serving 192.168.3.0/24 and 192.168.4.0/24)
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2 NAT rules in firewall with source IP address filter
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2 default IP routes with source IP address filter
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2 bridges … very similar setup (LAN IP address and DHCP server on the bridges)
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bridge with VLAN also similar
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bridge without VLAN or just one ethernet … a bid more complicated to handle the DHCP leases , as there is only one DHCP server
You can have a smarter setup, with preferred and failover PPPoE paths (but they probably fail both at the same time) http://forum.mikrotik.com/t/advanced-routing-failover-without-scripting/136599/1
There are many variations on this. Some ideas here … https://mum.mikrotik.com/presentations/US12/tomas.pdf