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DES-7200 Configuration Guide Chapter 2 Configuring BGP IP VPN
2-55
Carrier protocol is the protocol used to encapsulate passenger protocol during the process of
GRE encapsulation. In the application scenario of MPLS VPN over GRE, the carrier protocol is
generally IPv4.
Source address and destination address
While encapsulating the passenger protocol, we need to know the source address and
destination address of carrier protocol, so that the encapsulated packets can be forwarded on
the carrier network. The abovementioned source address and destination address are the
source address and destination address of GRE tunnel.
Tunnel endpoint
When packets are transported on the tunnel, there is always one device carrying out carrier
protocol encapsulation and another device carrying out de-encapsulation. The passenger
protocol information can only be known and handled by these two devices, while other carrier
network devices between them are unaware of the existence of passenger protocol. These two
devices are the endpoints of GRE tunnel.
2.3.4.2 Working principle
In the traditional MPLS VPN, private-network traffic carrying inner-layer VPN label and
outer-layer public-network label reaches the peer PE by means of label switching. When
non-MPLS network exists in the backbone network, the LSP will become discontinuous. The
GRE tunnel can help MPLS packets cross non-MPLS domain and realize continuous LSP.
GRE tunnel is a tunneling mechanism in IP network and support GRE with MPLS as the
passenger protocol, so that two devices on both sides of the IP network can exchange MPLS
packets. Considering GRE tunnel as a point-to-point logical link, devices at both ends of the
tunnel directly establish IGP adjacency and LDP adjacency on this link to distribute routes and
labels for LSP, while GRE tunnel becomes one hop of LSP.
MPLS as a passenger protocol
Take MPLS as the GRE tunnel of passenger protocol so that two devices interconnected through
non-MPLS network can forward MPLS packets to each other. After label operation at one end of
the tunnel, MPLS packets are encapsulated and then transported over the carrier network to the
other end of tunnel; label switching is then carried out after packet de-encapsulation at the other
end of tunnel. Fig 17 shows the format of encapsulated packets with IPv4 being the carrier
protocol and MPLS being the passenger protocol.