Routing issue. Some tips on troubleshooting required

Hi Folks

For some reason when i try to reach a particular VLAN from any machine in another VLAN it is looking for that machine on the ISP Gateway. Even on PFSENSE server if i try to ping a machine on that vlan the request seems to be going to the ISP gateway and failing. This was working fine in the morning. The only thing i played around was with the DNS settings. But even after rolling back the change in DNS server the issue is not resolved.

Please advice on how i troubleshoot this. This seems to be happening only for 1 VLAN

Ok I identified the problem But i cannot resolve it. I check the route table in PFSense (Diagnostics–> Routes) and found that all the machines in the specific VLAN having this issue was having an entry there and point to the Internet Gateway. Deleting these route entries resolves the issue, but these entries come back after a few minutes. These machines on this VLAN are my 2 DNS servers

Any ideas why these entries keep coming back ?

I think i resolved the issue. One change i had done was on my ISP router i set the DNS servers as my internal DNS servers to ensure that even if anyone connects to the ISP router directly bypassing PFSENSE they will be forced back to my internal DNS server thereby enforcing the filters . However by doing that i guess i created a direct link between the Internet Gateway and the DNS servers which caused the automatic entry in the route table of PFSENSE. For now i have changed the DNS entries in the ISP router and deleted the route entries in PFSENSE and its been 30 minutes and those entries have not come back in the route table.