Manual:Load balancing multiple same subnet links

Applies to RouterOS: v4,v5

Warning: Article is migrated to our new documentation!

Summary

This example demonstrates how to set up load balancing if provider is giving IP addresses from the same subnet for all links.

Application Example

Provider is giving us two links with IP addresses from the same network range (10.1.101.10/24 and 10.1.101.18/24). Gateway for both of these links is the same 10.1.101.1

Configuration Example 1

Here is the whole configuration for those who want to copy&paste

/ip address
add address=10.1.101.18/24 interface=ether1 
add address=10.1.101.10/24 interface=ether2
add address=192.168.1.1/24 interface=Local
add address=192.168.2.1/24 interface=Local

/ip route
add gateway=10.1.101.1
add gateway=10.1.101.1%ether1 routing-mark=first
add gateway=10.1.101.1%ether2 routing-mark=other

/ip firewall nat
add action=masquerade chain=srcnat out-interface=ether1
add action=masquerade chain=srcnat out-interface=ether2

/ip firewall mangle
add action=mark-routing chain=prerouting src-address=192.168.1.0/24 new-routing-mark=first
add action=mark-routing chain=prerouting src-address=192.168.2.0/24 new-routing-mark=other

IP address

In previous RouterOS version, multiple IP addresses from the same subnet on different interfaces were not allowed. Fortunately, from v4 RouterOS allows such configurations.

In this example our provider assigned two upstream links, one connected to ether1 and other to ether2. Our local network has two subnets 192.168.1.0/24 and 192.168.2.0/24

/ip address
add address=10.1.101.18/24 interface=ether1 
add address=10.1.101.10/24 interface=ether2
add address=192.168.1.1/24 interface=Local
add address=192.168.2.1/24 interface=Local

After IP address is set up, connected route will be installed as ECMP route

[admin@MikroTik] /ip route> print detail
 0 ADC  dst-address=10.1.101.0/24 pref-src=10.1.101.18 gateway=ether1,ether2 
        gateway-status=ether1 reachable,ether2 reachable distance=0 scope=10

Mangle and NAT

In our example very simple policy routing is used. Clients from 192.168.1.0/24 subnet is marked to use "first" routing table and 192.168.2.0/24 to use "other" subnet.

Note: The same can be achieved by setting up route rules instead of mangle.

/ip firewall mangle
add action=mark-routing chain=prerouting src-address=192.168.1.0/24 new-routing-mark=first
add action=mark-routing chain=prerouting src-address=192.168.2.0/24 new-routing-mark=other

And masquerade our local networks

/ip firewall nat
add action=masquerade chain=srcnat out-interface=ether1
add action=masquerade chain=srcnat out-interface=ether2

Warning: You will also have to deal with traffic coming to and from the router itself. For explanations look at PCC configuration example.

IP route

We are adding two gateways, one to resolve in "first" routing table and another to "other" routing table.

/ip route
add gateway=10.1.101.1%ether1 routing-mark=first
add gateway=10.1.101.1%ether2 routing-mark=other

Interesting part of these routes is how we set gateway. gateway=10.1.101.1%ether1 means that gateway 10.1.101.1 will be explicitly reachable over ether1

[admin@MikroTik] /ip route> print detail 
Flags: X - disabled, A - active, D - dynamic, 
C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme, 
B - blackhole, U - unreachable, P - prohibit 
 0 A S  dst-address=0.0.0.0/0 gateway=10.1.101.1%ether2 
        gateway-status=10.1.101.1 reachable ether2 distance=1 scope=30 
        target-scope=10 routing-mark=other 

 1 A S  dst-address=0.0.0.0/0 gateway=10.1.101.1%ether1 
        gateway-status=10.1.101.1 reachable ether1 distance=1 scope=30 
        target-scope=10 routing-mark=first 

Finally, we have one additional entry specifying that traffic from the router itself (the traffic without any routing marks) will be resolved in the main routing table.

/ip route
add gateway=10.1.101.1