H3C Technologies H3C SecPath F1000-E - Manual (Page 34)

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Figure 18 Learning tunnel destination addresses dynamically
Different from a P2P GRE tunnel interface, a P2MP GRE tunnel interface does not require manual
configuration of the tunnel destination addresses but learns them from GRE tunnel packets received from
peers. As shown in Figure 18 , Device A resides at the headquarters and has a P2MP GRE tunnel
interface configured, while Device B resides at a branch and has a P2P GRE tunnel interface configured.
After Device A receives a GRE packet from Device B, it establishes a tunnel entry, taking the source
address in the transport protocol (IPv4) header as the tunnel destination address and the source address
in the passenger protocol (IPv4) header (that is, the private network address of the branch) as the packet
destination address.
When forwarding a packet through a P2MP GRE tunnel, the device searches the tunnel entries for the
tunnel destination address according to the packet’s destination address, and then encapsulates the
packet with GRE and then with IPv4, using the tunnel destination address as the destination address in
the transport protocol header.
NOTE:
The mask length of the packet destination address in a tunnel entry is configurable. After you configure a
mask length for a packet destination address, the node at the headquarters establishes only one tunnel
entry for private IP addresses in the same network segment, thus reducing the number of tunnel entries on
the node at the headquarters and allowing branches to initiate establishment of tunnels by sending
emulated data to the node at the headquarters.
10.2.1.2/24 10.1.1.2/24
Tunnel0
10.3.1.1/24
11.1.1.1/24 11.1.1.2/24
IPv4 network
GRE tunnel
Tunnel0
10.3.1.2/24
Device A Device B
Host A Host B
Headquarters Branch 10.1.1.2 GRE 11.1.1.2
Tun Dest
11.1.1.2
Dest
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