GRE application scenarios
The following shows typical GRE application scenarios:
Connecting networks running different protocols over a single backbone
As shown in Figure 95, IPv6 network 1 and IPv6 network 2 are IPv6 networks, and IPv4 network 1 and IPv4 network 2 are IPv4 networks. Through the GRE tunnel between Device A and Device B, IPv6 network 1 can communicate with IPv6 network 2 and IPv4 network 1 can communicate with IPv4 network 2, without affecting each other.
Figure 95: Network diagram
Enlarging network scope
In an IP network, the maximum TTL value of a packet is 255. If two devices have more than 255 hops in between, they cannot communicate with each other. By using a GRE tunnel, you can hide some hops to enlarge the network scope. As shown in Figure 96, only the tunnel-end devices (Device A and Device D) of the GRE tunnel are counted in hop count calculation. Therefore, there are only three hops between Host A and Host B.
Figure 96: Network diagram
Constructing VPN
As shown in Figure 97, Site 1 and Site 2 both belong to VPN 1 and are located in different cities. Using a GRE tunnel can connect the two VPN sites across the WAN.
Figure 97: Network diagram
