GRE Tunnel interface Overview

The GRE (Generic Routing Encapsulation) tunnel interface is a virtual point-to-point connection between two routers that can communicate. It is also known simply as a tunnel interface.

Overview of the GRE Tunnel
Overview of the GRE Tunnel

The GRE tunnel interface is used for Internet VPN and other applications. It can also be used for IPv6 communication over an IPv4 network.

A multipoint GRE tunnel interface is also available. The default GRE tunnel interface is point-to-point.

Configuring the GRE Tunnel Interface

The steps for configuring the GRE tunnel interface are as follows

  1. Creating a Tunnel Interface
  2. Configuring tunnel destination/tunnel source
  3. Configuring IP address

Creating a Tunnel Interface

To create the tunnel interface, use the following command in global configuration mode.

Creating a Tunnel Interface

(config)#interface Tunnel <num>
(config-if)#

<num> : interface number

Configuring tunnel destination/tunnel source

Next, specify the IP address of the forwarding IPv4 header. tunnell destination command specifies the destination IP address of the forwarding IPv4 header and tunnell source command specifies the source IP address of the forwarding IPv4 header.

Configuring tunnel destination/tunnel source

(config)#interface Tunnel <num>
(config-if)#tunnel destination <dst-ip-address>
(config-if)#tunnel source <source-ip-address|source-interface>

<num> : interface number
<dst-ip-address> : Destination IP address of the encapsulating IP header
<source-ip-address> : Source IP address of the encapsulating IP header
<source-interface> : Interface to be used as the source IP address of the encapsulating IP header

If the IP address specified by the tunnel destination can be reached, the tunnel interface will be in the up/up state. However, both routers on which you configure tunnels must have the other party’s tunnel destination set to their own tunnel source and the other party’s tunnel source set to their own tunnel destination. If the tunnel destination and tunnel source configurations are correct, both routers with tunnels configured will look as if they are directly connected point-to-point.

Configuring tunnel destination/tunnel source
Configuring tunnel destination/tunnel source

Configuring IP address

An IP address is required for IP communication on the tunnel interface. The tunnel interface is configured with an IP address just like a normal interface. As I have repeatedly stated, remember that you can only connect to the network and communicate with it when you configure the IP address.

Configuring IP address

(config)#interface Tunnel <num>
(config-if)#ip address <address> <subnetmask>

<num> : interface number
<address> : IP address
<subnetmask> : subnet mask

The IP address of the tunnel interface is often configured to use the IP address of another interface by ip unnumbered command.

To send and receive IPv6 packets over the tunnel interface, you must configure an IPv6 address.

Forwarding IP packets from the GRE tunnel interface

IP packets sent from the tunnel interface are encapsulated in GRE headers and IPv4 headers for forwarding. The destination/source IP address of the forwarding IPv4 header is the IP address configured in the tunnel destination/tunnel source. It is then output from the associated physical interface. The associated physical interface is determined from the routing table and tunnel destination. In addition, when the output from the physical interface is sent out, it is encapsulated in a Layer 2 protocol, such as Ethernet, that is appropriate for the physical interface. Finally, the output is sent out as a physical signal, such as an electrical signal, to the transmission medium.

Figure Sending packets from the tunnel interface
Figure Sending packets from the tunnel interface

When the GRE tunnel goes down

If there is no connectivity to the tunnel destination of the tunnel interface, the tunnel interface will be in an up/down state, as shown in the following command output.

R1#show ip interface brief
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            192.168.1.1     YES NVRAM  up                    up
FastEthernet0/1            192.168.0.1     YES NVRAM  up                    up
Loopback0                  10.1.1.1        YES NVRAM  up                    up
Tunnel13                   172.16.1.1      YES manual up                    down

Make sure that connectivity to the tunnel destination is properly established. By default, connectivity to the tunnel destination is verified based on the routing table.

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