Hello David,
you cannot run the same routing protocol inside and outside the tunnel
router ospf 2020
log-adjacency-changes
network 10.16.0.0 0.0.0.3 area 0
network 172.15.1.0 0.0.0.255 area 0
network 172.15.2.0 0.0.0.255 area 0
network 172.15.3.0 0.0.0.255 area 0
>>>> network 192.168.0.0 0.0.0.3 area 0
!
you need to use a different protocol over the tunnel for example EIGRP or a different OSPF routing process
You are facing a recursive routing error: once the tunnel is up also the tunnel endpoint addresses are advertised over it and this is not good at all
remove the line network 192.168.0.0 0.0.0.3 area 0 on both routers R3 and R4
use a different process like
router ospf 123
network 192.168.0.0 0.0.0.3 area 0
network 192.168.123.3 0.0.0.0 area 0
to see everything is working well you need to advertise at least one IP subnet on each side
in loop3
ip address 192.168.123.3 255.255.255.255
no shut
a similar setup has to be done on R4:
router ospf 123
network 192.168.0.0 0.0.0.3 area 0
network 192.168.123.4 0.0.0.0 area 0
in loop3
ip address 192.168.123.4 255.255.255.255
no shut
Hope to help
Giuseppe
