Google Cloud doesn’t allow certain IP protocols, such as egress traffic on TCP port 25 within a VPC network. For more information, see always blocked traffic.
Certain GRE traffic (beta)
• Traffic in Cloud VPN tunnels
• Traffic on Cloud Interconnect attachments (VLANs)
• Traffic for forwarding rules (load balacing or protocol forwarding)
GRE is allowed within a VPC network
Protocols other than TCP, UDP, ICMP, AH, ESP, SCTP, and GRE to external IP addresses of Google Cloud resources The type of resource further limits the protocol. For example, Network TCP/UDP Load Balancing supports only TCP and UDP. Also, a forwarding rule for protocol forwarding only processes a single protocol. Refer to the protocol forwarding documentation for a list of supported protocols.
Egress traffic to TCP destination port 25 (SMTP)
• instances to external IP addresses on the internet
• instances to external IP addresses of instances