R3303-HP HSR6800 Routers Layer 3 - IP Services Configuration Guide

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GRE application scenarios
The following shows typical GRE application scenarios:
Connecting private networks running different protocols over a single backbone
As shown in Figure 110, Group 1 and Group 2 are IPv6 networks, and Team 1 and Team 2 are IPv4
networks. Through the GRE tunnel between Router A and Router B, Group 1 can communicate with
Group 2 and Team 1 can communicate with Team 2, without affecting each other.
Figure 110 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 111, onl
y the tunnel-end devices (Router A and Router D)
of the GRE tunnel are counted in hop count calculation. Therefore, there are only three hops between
Host A and Host B.
Figure 111 Network diagram
Constructing VPN
As shown in Figure 112, 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.