If users are experiencing intermittent access but network connectivity is consistent, it may indicate a DNS resolution problem. When traffic exits out a particular WAN interface, the source IP address may be changed to properly route over that WAN line. ISP DNS servers check for this and typically do not respond to DNS queries coming from a different carrier.
Symptoms include: being able to ping an IP address but not a hostname; Internet access works over WAN line but not the other; or users having to reload web pages to display it properly.
To test, check the DNS server addresses on one of the affected computers. These addresses should be changed to public DNS servers such as 188.8.131.52, 184.108.40.206, or 220.127.116.11. These servers typically respond regardless of source. Test Internet connectivity again to see if the issue is resolved.
If changing the DNS servers resolved the issue, please check if:
1. The firewall or DHCP server is configured with the DNS servers of one of the ISPs.
2. The internal DNS servers are configured with "forwarders," upstream DNS servers, that relay public DNS requests to the ISP's DNS servers.
In either case, the DNS configuration will need to be updated to use DNS servers other than the ISP DNS servers.
If changing the DNS server addresses did not resolve the issue, please contact our Technical Support team for further assistance.