BGP or OSPF over ipsec tunnels?

[JAHMS]

I've been reading fortigate secure sd-wan admin guide and other contents on overlays and they all or mostly recommend to use BGP as the routing protocol between sites. I only have 3 sites (hub and spokes) in same country communicating via an ipsec tunnel and I feel like BGP is for big companies with multiple sites accross the globe and does not apply to my case. I was inclined to use OSPF, and the tunnels will be in area O. 

What do you think?

[Toshi_Esumi]

OSPF might not scale well to a large scale network as Google AI states:
" OSPF nodes within the same area share the same complete network topology information for that specific area This is because each router in an area maintains an identical Link-State Database (LSDB) built from Link-State Advertisements (LSAs) flooded throughout that area. This database contains all the details of the area's topology, enabling each router to run the Shortest Path First (SPF) algorithm and calculate a loop-free path."
It gets more CPU intensive quickly when the network grows.

On the other hand BGP is more scalable because it is designed for large-scale, inter-network routing on the internet. It doesn't require a huge database and calculations unlike OSPF. This means BGP also works very well in a small network like two or three nodes. BGP is easier to troubleshoot because it uses unicast messages, node-to-node (neighbors) even when multiple nodes are on the same broadcast network, while OSPF uses multi-cast messages to communicate.

So I always prefer BGP regardless of the network size because of above. But I'm not against using OSPF if you're much more familiar than BGP. I'm just less familiar with OSPF because I don't have many opportunities to use it.

Toshi