My setup was similar when I didn't have our datacenter network behind the (non Meraki MX) firewall.  My MS425s were doing OSPF between themselves and Cisco ASRs, and whatever the datacenter vendor had on their end.  I used a transport network, as you mentioned, between the Meraki and ASR, and then another transport network between the ASR and the datacenter vendor's equipment.  I did not extend the internal VLANs beyond the MS425s, but I did advertise the subnets contained within the VLANs to OSPF so everything participating in the OSPF area had the information about which router had (for example) the 10.1.2.0/24 network.   It took a bit of fiddling the first time to get it to work, but the problems ended up being speed/duplex between the Meraki gear and the ASR.  The aforementioned MS425s replaced an old Catalyst 6509.   I have now moved the datacenter network behind the firewall, as we bought some larger firewall boxes, so now FW vendor P is handling OSPF.  But I would not hesitate to use OSPF on Meraki again if I had a need for it.   ... View more