F3726, F3211, F3174, R5135, R3816-HP Firewalls and UTM Devices VPN Configuration Guide-6PW100

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calculates a checksum for the received packet and compares it with that carried in the packet. If the
checksums are the same, the receiver considers the packet intact and continues to process the
packet. Otherwise, the receiver discards the packet.
95BGRE application scenarios
The following shows typical GRE application scenarios:
324BConnecting private networks running different protocols over a single backbone
As shown in 667HFigure 4, Group 1 and Group 2 are IPv6 networks, and Team 1 and Team 2 are IPv4
networks. Through the GRE tunnel between Device A and Device B, Group 1 can communicate with
Group 2 and Team 1 can communicate with Team 2, without affecting each other.
Figure 4 Network diagram
325BEnlarging 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
668HFigure 5, 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 5 Network diagram
326BConstructing VPN
As shown in 669HFigure 6, 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.
IPv6 protocol
Group 1
Internet
IP protocol
Team 1
IPv6 protocol
Group 2
IP protocol
Team 2
Device A Device B
GRE tunnel
Device A
Device B Device C
Device D
Host A Host B
IP network
GRE tunnel
IP network
IP network