Panasonic NN46240-710 - Manual (Page 56)

2 GRE troubleshooting
Nortel Secure Router 8000 Series
_________ Troubleshooting - VPN
2.1 GRE overview
This section covers the following topics:
• Introduction to GRE
• Related concepts of GRE
• Applications of GRE
2.1.1 Introduction to GRE
Generic Routing Encapsulation (GRE) encapsulates packets of any network layer, such as
Internetwork Packet Exchange (IPX), to enable their transmission by another network layer
protocol such as IP.
GRE is a Layer 3 virtual private network (VPN) tunneling protocol that provides a tunnel for
transparent transmission of various protocol packets.
A tunnel is a virtual point-to-point connection, and it can be considered as a virtual interface
that supports point-to-point (P2P) connections only. The virtual interface provides a path for the
transmission of data that can be encapsulated and decapsulated at both ends of the path.
2.1.2 Related concepts of GRE
Encapsulation and decapsulation of packets
Payload refers to the datagram received by the system for encapsulation and routing. The
payload is first added with a GRE header. That is, the payload is encapsulated into GRE packets.
The GRE packets are then encapsulated into IP packets to be transported over the IP layer.
Figure 2-1 shows the format of an encapsulated tunnel packet.
Figure 2-1 Format of an encapsulated tunnel packet
Delivery Header ( Transport Protocol )
GRE Header ( Encapsulation Protocol )
Payload Packet ( Passenger Protocol )
Transmission of packets
The transmission of packets in the tunnel involves encapsulation and decapsulation. Use Figure
2-2 as an example to describe the process.
2-2 Nortel Networks Inc. Issue 5.3 (30 March2009)