Thank you for the answer. The IPs are both ends of a VPN tunnel. Source IP is the local gateway external IP and the destination is the peer gateway IP.
The IP packet is forwarded to the right interface, so to my understanding the layer 2 part is working. we see no ethertype ARP requests to know the IP.
Still, the packet is forwarded to the outgoing interface, but with a layer 2 broadcast MAC as destination.
Other VPNs are running fine via the same default gateway, the only difference is the peer IP, and for one specific IP, a layer 2 broadcast is send.
This is an example of something that works:
10:14:08.809013 00:12:c1:ce:90:08 ^ 00:00:5e:00:01:33, ethertype IPv4 (0x0800), length 134: 198.45.128.10 ^ 64.62.2.253: ESP(spi=0xce785fc9,seq=0x1f), length 100 (IPs changed)
The question is more, if there is any situation where the packet below is valid according to RFC. I have a ticket with Checkpoint but I wanted to ask here also.
10:12:10.207381 00:12:c1:ce:90:08 ^ Broadcast, ethertype IPv4 (0x0800), length 134: x.x.x.x ^ x.x.x.x: ESP(spi=0x78e02e92,seq=0xb7)