Load Balancing Multi gateway

Quick Start for Impatient

Configuration export from the gateway router:
/ ip address
add address=192.168.0.1/24 network=192.168.0.0 broadcast=192.168.0.255 interface=Local comment="" \
disabled=no
add address=10.111.0.2/24 network=10.111.0.0 broadcast=10.111.0.255 interface=wlan2 \
comment="" disabled=no
add address=10.112.0.2/24 network=10.112.0.0 broadcast=10.112.0.255 interface=wlan1 \
comment="" disabled=no
/ ip firewall mangle
add chain=prerouting in-interface=Local connection-state=new nth=1,1,0 \
action=mark-connection new-connection-mark=odd passthrough=yes comment="" \
disabled=no
add chain=prerouting in-interface=Local connection-mark=odd action=mark-routing \
new-routing-mark=odd passthrough=no comment="" disabled=no
add chain=prerouting in-interface=Local connection-state=new nth=1,1,1 \
action=mark-connection new-connection-mark=even passthrough=yes comment="" \
disabled=no
add chain=prerouting in-interface=Local connection-mark=even action=mark-routing \
new-routing-mark=even passthrough=no comment="" disabled=no
/ ip firewall nat
add chain=srcnat connection-mark=odd action=src-nat to-addresses=10.111.0.2 \
to-ports=0-65535 comment="" disabled=no
add chain=srcnat connection-mark=even action=src-nat to-addresses=10.112.0.2 \
to-ports=0-65535 comment="" disabled=no
/ ip route
add dst-address=0.0.0.0/0 gateway=10.111.0.1 scope=255 target-scope=10 routing-mark=odd \
comment="" disabled=no
add dst-address=0.0.0.0/0 gateway=10.112.0.1 scope=255 target-scope=10 routing-mark=even \
comment="" disabled=no
add dst-address=0.0.0.0/0 gateway=10.112.0.1 scope=255 target-scope=10 comment="" \
disabled=no
Explanation

First we give a code snippet and then explain what it actually does.
Mangle
/ ip address
add address=192.168.0.1/24 network=192.168.0.0 broadcast=192.168.0.255 interface=Local comment="" \
disabled=no
add address=10.111.0.2/24 network=10.111.0.0 broadcast=10.111.0.255 interface=wlan2 \
comment="" disabled=no
add address=10.112.0.2/24 network=10.112.0.0 broadcast=10.112.0.255 interface=wlan1 \
comment="" disabled=no

The router has two upstream (WAN) interfaces with the addresses of 10.111.0.2/24 and 10.112.0.2/24. The LAN interface has the name "Local" and IP address of 192.168.0.1/24.
/ ip firewall mangle

add chain=prerouting in-interface=Local connection-state=new nth=1,1,0 \
action=mark-connection new-connection-mark=odd passthrough=yes comment="" \
disabled=no


First we take every second packet that establishes new session (note connection-state=new), and mark it with connection mark "odd". Consequently all successive packets belonging to the same session will carry the connection mark "odd". Note that we are passing these packets to the second rule (passthrough=yes) to place a routing mark on these packets in addition to the connection mark.
add chain=prerouting in-interface=Local connection-mark=odd action=mark-routing \
new-routing-mark=odd passthrough=no comment="" disabled=no

The rule above places the routing mark "odd" on all packets that belong to the "odd" connection and stops processing all other mangle in prerouting chain rules for these packets.
add chain=prerouting in-interface=Local connection-state=new nth=1,1,1 \
action=mark-connection new-connection-mark=even passthrough=yes comment="" \
disabled=no
add chain=prerouting in-interface=Local connection-mark=even action=mark-routing \
new-routing-mark=even passthrough=no comment="" disabled=no

These rules do the same for the remaining half of the traffic as the first two rules for the first half of the traffic.

The code above effectively means that each new connection initiated through the router from the local network will be marked as either "odd" or "even" with both routing and connection marks.
NAT
/ ip firewall nat
add chain=srcnat connection-mark=odd action=src-nat to-addresses=10.111.0.2 \
to-ports=0-65535 comment="" disabled=no
add chain=srcnat connection-mark=even action=src-nat to-addresses=10.112.0.2 \
to-ports=0-65535 comment="" disabled=no

All traffic marked "odd" is being NATted to source IP address of 10.111.0.2, while traffic marked "even" gets "10.112.0.2" source IP address.
Routing
/ ip route
add dst-address=0.0.0.0/0 gateway=10.111.0.1 scope=255 target-scope=10 routing-mark=odd \
comment="" disabled=no
add dst-address=0.0.0.0/0 gateway=10.112.0.1 scope=255 target-scope=10 routing-mark=even \
comment="" disabled=no
add dst-address=0.0.0.0/0 gateway=10.112.0.1 scope=255 target-scope=10 comment="" \
disabled=no comment="gateway for the router itself"

For all traffic marked "odd" (consequently having 10.111.0.2 translated source address) we use 10.111.0.1 gateway. In the same manner all traffic marked "even" is routed through the 10.112.0.1 gateway. Finally, we have one additional entry specifying that traffic from the router itself (the traffic without any routing marks) should go to 10.112.0.1 gateway.