What is the function of the tunnel interface (GRE interface)

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The tunnel interface (GRE interface) encapsulates and decapsulates data packets using GRE. The tunnel interface that sends encapsulated packets is called the tunnel source interface, and the one that receives these packets on the peer end is called the tunnel destination interface.
Generally, the local WAN interface is used as the tunnel source interface, and the peer WAN interface is used as the tunnel destination interface.

Other related questions:
What is the function of the tunnel interface (GRE interface)
The tunnel interface (GRE interface) encapsulates and decapsulates data packets using GRE. The tunnel interface that sends encapsulated packets is called the tunnel source interface, and the one that receives these packets on the peer end is called the tunnel destination interface. Generally, the local WAN interface is used as the tunnel source interface, and the peer WAN interface is used as the tunnel destination interface.

Types of interfaces on both ends of the GRE tunnel for the USG6000
Interfaces on both ends of the GRE tunnel are tunnel interfaces, used to encapsulate and decapsulate packets. The physical interface used to transmit encapsulated packets is known as the tunnel source interface, and the peer interface used to receive the packets is known as the tunnel destination interface.

Can the MTU of the GRE tunnel interface take effect
If you set an MTU value on a GRE tunnel interface, forwarding of data packets through the GRE tunnel will be affected. If the packet length exceeds the MTU value on the tunnel interface, the device fragments the packet.

What are the functions of tunnel interface IP addresses
An tunnel interface IP address enables the tunnel interface to become Up and is mandatory. The IP address is not involved in packet encapsulation. When static routes are used to direct traffic to a GRE tunnel, the IP address of the interfaces at both ends of the tunnel can belong to different network segments. When an GRE tunnel is established for OSPF routing exchange between two networks connected to the tunnel, the IP address of the interfaces at both ends of the tunnel must belong to the same network segment. Otherwise, the tunnel interfaces fail to establish an OSPF adjacency.

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