Following is a complete script for Mikrotik to combine/load balance two DSL lines. In this example I used MikrotikT RB750 5 ports router.
2 ports were connected with two difference DSL Routers,
and 3rd port was connected with User LAN.
Both DSL are of same speed , i.e 10Mb each.
DSL MODEM IP’S
DSL MODEM 1 = 192.168.1.1
DSL MODEM 2 = 192.168.2.1
Also don’t forget to rename the interface names accordingly.
In my personnel experience , If users request are directly hitting Mikrotik configured with PCC , then you will get good load balancing. Use src-address as classifier, this way you will get rid of problems like https/broken link, streaming issues etc. Load balancing using this PCC technique (src-address) will be effective and balanced approach when more and more connections (from clients) that occurred. I also recommend to use SQUID proxy server along with mikrotik , either parallel or in front or backend , for better response time and it will also increase good browsing experience to users.
If somehow you are not satisfied with the src-address approach,play with the PCC-Classifier, then Try both addresses and ports as the classifier. While this will randomize things the most and in theory give you the most fair allocation of bandwidth, BUT there is also a good chance that it will break certain things like banking web sites and some forums. This is because often times a HTTP requests will generate several connections, so there is a chance that some requests may go out a different route than the initial one, and that will break secure web sites. For that reason I usually stick with src-address for PCC load balancing.
add address=192.168.0.1/24 network=192.168.0.0 broadcast=192.168.0.255 interface=Local
add address=192.168.1.2/24 network=192.168.1.0 broadcast=192.168.1.255 interface=WAN1
add address=192.168.2.2/24 network=192.168.2.0 broadcast=192.168.2.255 interface=WAN2
/ip dns set allow-remote-requests=yes cache-max-ttl=1w cache-size=5000KiB max-udp-packet-size=512 servers=126.96.36.199
/ip firewall mangle
add chain=input in-interface=WAN1 action=mark-connection new-connection-mark=WAN1_conn
add chain=input in-interface=WAN2 action=mark-connection new-connection-mark=WAN2_conn
add chain=output connection-mark=WAN1_conn action=mark-routing new-routing-mark=to_WAN1
add chain=output connection-mark=WAN2_conn action=mark-routing new-routing-mark=to_WAN2
add chain=prerouting dst-address=192.168.1.0/24 action=accept in-interface=Local
add chain=prerouting dst-address=192.168.2.0/24 action=accept in-interface=Local
add chain=prerouting dst-address-type=!local in-interface=Local per-connection-classifier=both-addresses-and-ports:2/0 action=mark-connection new-connection-mark=WAN1_conn passthrough=yes
add chain=prerouting dst-address-type=!local in-interface=Local per-connection-classifier=both-addresses-and-ports:2/1 action=mark-connection new-connection-mark=WAN2_conn passthrough=yes
add chain=prerouting connection-mark=WAN1_conn in-interface=Local action=mark-routing new-routing-mark=to_WAN1
add chain=prerouting connection-mark=WAN2_conn in-interface=Local action=mark-routing new-routing-mark=to_WAN2
add dst-address=0.0.0.0/0 gateway=192.168.1.1 routing-mark=to_WAN1 check-gateway=ping
add dst-address=0.0.0.0/0 gateway=192.168.2.1 routing-mark=to_WAN2 check-gateway=ping
add dst-address=0.0.0.0/0 gateway=192.168.1.1 distance=1 check-gateway=ping
add dst-address=0.0.0.0/0 gateway=192.168.2.1 distance=2 check-gateway=ping
/ip firewall nat
add chain=srcnat out-interface=WAN1 action=masquerade
add chain=srcnat out-interface=WAN2 action=masquerade