How GRE Works

GRE in tunneling (packets transmitted in the GRE tunnel have 3 IP addresses)
1, establish a virtual direct-connect route, can be considered as a tunnel, analog link

1, receive the original IP packet as a passenger agreement, the IP address of the original IP packet header is a private IP address
2, the original IP Packet Encapsulation GRE protocol, the GRE protocol is called Encapsulation Protocol, the encapsulated packet header IP address is a virtual direct link

IP addresses at both ends
3, the entire GRE data packet as data, the outer layer package public network IP header, that is, the origin and end of the tunnel, thereby

Route to tunnel endpoint


The GRE tunnel only supports routers, does not support concentrators and the PIX, and the ASA

The interface status of the GRE tunnel is down
1. No route to tunnel endpoint address
2. The route to the tunnel end address points to the tunnel interface itself
3, tunnel origin address of the interface state is down


GRE keepalive Default 10s sends a maintenance neighbor relationship for 3 consecutive packets without responding to link protocol status change

Down only supports point-to-point

How GRE Works