In general, The connection should be like PC---SW---ROUTER---Many ISP router---Router---SW---PC
so the packet will be from layer 2 ---layer 3---layer 2
What if I put one layer 2 switch in the network
PC---SW---ROUTER---MANY ISP ROUTER---LAYER 2 SW---MANY ISP ROUTER---ROUTER---SW---PC
Can any network form between 2 PC? What should be configured on the layer 2 SW in the middle of the network ? I would say default gateway is not enough ? Of course , no normal people would do this ,but what-if ?