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I had the exact same issue before, but it happened on pfSense.
Turns out that there was conflict of DHCP between the pfSense and Proxmox SDN.
If setting up Subnet, Gateway, DHCP range in VNet and automatic DHCP in Zone. Then the SDN will become a DHCP too.
After restarting the firewall, the guest VMs get IP from pfSense DHCP, but if restarting the guest VMs, the VMs go to get IP from SDN DHCP.
Assume that the gateway IP settings are the same for both the firewall and SDN. You never know which DHCP is used by the VMs unless you go to check the lease record.
When you found that the VMs couldn't access the Internet, you tried to ping the gateway and it was successful. However, at that moment in time, you were actually pinging the gateway of SDN.
I think your OPNSense had set up NAT for VMs to access the Internet, but not the SDN. That was why when the VMs get their IP from OPNSense can go to the Internet but failed when they got the IPs from SDN.
Turns out that there was conflict of DHCP between the pfSense and Proxmox SDN.
If setting up Subnet, Gateway, DHCP range in VNet and automatic DHCP in Zone. Then the SDN will become a DHCP too.
After restarting the firewall, the guest VMs get IP from pfSense DHCP, but if restarting the guest VMs, the VMs go to get IP from SDN DHCP.
Assume that the gateway IP settings are the same for both the firewall and SDN. You never know which DHCP is used by the VMs unless you go to check the lease record.
When you found that the VMs couldn't access the Internet, you tried to ping the gateway and it was successful. However, at that moment in time, you were actually pinging the gateway of SDN.
I think your OPNSense had set up NAT for VMs to access the Internet, but not the SDN. That was why when the VMs get their IP from OPNSense can go to the Internet but failed when they got the IPs from SDN.
