This is the basics of routing.
You don't need to use source based route.
For example, let's assume you have the 192.168.10.0/24 network on MX1 and the 192.168.20.0/24 network on MX2.
The first thing is to create a link vlan on both MXs, for example the 10.10.10.0/30 network (VLAN 99) where MX1 will have IP 10.10.10.1 and MX2 will have IP 10.10.10.2.
If MX1 wants to reach MX2's network, just create a route pointing to the link IP of MX2 as the next hop.
So it would look like this, to reach the network 192.168.20.0/24 the next hop is IP 10.10.10.2
Very simple, for the MX2 just do it in reverse, to get to the 192.168.10.0/24 network the next hop is 10.10.10.1
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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