GRE packet encapsulation process on the USG6000

1. Upon receiving an IP packet over an interface connected to the IP network, the firewall enables the IP processing part to process the IP packet. 2. The IP processing part checks the destination address in the packet header to determine the forwarding mode. If the packet needs to pass through the GRE tunnel to arrive at the destination, the IP processing part sends the packet to the corresponding tunnel interface. 3. Upon receiving the packet, the tunnel interface encapsulates the packet with a GRE packet header and then returns the packet to the IP processing part. 4. The IP processing part encapsulates the GRE packet with a new IP packet header (the source address is the tunnel source interface IP address and the destination address is the tunnel destination interface IP address), and forwards the encapsulated IP packet over the physical port connected to the Internet based on the destination address and routing table.

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