10.2. Configuring IPv4 GRE Tunnel

Function Introduction

Tunneling is an encapsulation technology, which uses one network protocol to encapsulate packet of another network protocol and transfer them over a virtual point to point connection. The virtual connection is called a tunnel. Tunneling refers to the whole process from data encapsulation to data transfer to data de-encapsulation.

Principle Description

When it is required to communicate with isolated IPv4 networks, you should create a tunnel mechanism between them. The tunnel with transmit protocol of gre connected with two isolated IPv4 island is called IPv4 gre tunnel, which is that IPv4 packets are encapsulated by gre protocol over outer IPv4 packets. Gre tunnel would add gre head in encapsulated packets, including key, sequence, checksum and so on. In order to make an implement of gre tunnel, both tunnel endpoints must support the IPv4 protocol stacks.

IPv4 gre tunnel processes packets in the following ways:

• A host in the IPv4 network sends an IPv4 packet to Switch1 at the tunnel source.

• After determining according to the routing table that the packet needs to be forwarded through the tunnel, Switch1 encapsulates the IPv4 packet with an IPv4 header and forwards it through the physical interface of the tunnel.

• Upon receiving the packet, Switch2 de-encapsulates the packet.

• Switch2 forwards the packet according to the destination address in the de-encapsulated IPv4 packet. If the destination address is the device itself, Switch2 forwards the IPv4 packet to the upper-layer protocol for processing. In the process of de-encapsulation, it would check gre key, only the matched key of packet can be processed, otherwise discarded.

The ip address of tunnel source and tunnel destination is manually assigned, and it provides point-to-point connection. By using overlay tunnels, you can communicate with isolated IPv4 networks without upgrading the IPv4 infrastructure between them. Overlay tunnels can be configured between border routers or between border routers and a host.

The primary use is for stable connections that require regular secure communication between two edge routers or between an end system and an edge router, or for connection to remote IPv4 networks, gre key is alternative configuration.

step 1 Enter the configure mode

Switch# configure terminal

step 2 Enter the interface configure mode and set the attributes of the interface

Interface configuration for Switch1:

Switch(config)# interface eth-0-1
Switch(config-if)# no switchport
Switch(config-if)# no shutdown
Switch(config-if)# ip address 192.168.10.1/24
Switch(config-if)# tunnel enable
Switch(config-if)# exit
Switch(config)# interface eth-0-2
Switch(config-if)# no switchport
Switch(config-if)# no shutdown
Switch(config-if)# ip address 192.168.11.1/24
Switch(config-if)# exit

Interface configuration for Switch2:

Switch(config)# interface eth-0-1
Switch(config-if)# no switchport
Switch(config-if)# no shutdown
Switch(config-if)# ip address 192.168.20.1/24
Switch(config-if)# tunnel enable
Switch(config-if)# exit
Switch(config)# interface eth-0-2
Switch(config-if)# no switchport
Switch(config-if)# no shutdown
Switch(config-if)# ip address 192.168.12.1/24
Switch(config-if)# exit

step 3 Configure the tunnel interface

Tunnel interface configuration for Switch1:

Switch(config)# interface tunnel1
Switch(config-if)# tunnel mode gre
Switch(config-if)# tunnel source eth-0-1
Switch(config-if)# tunnel destination 192.168.20.1
Switch(config-if)# tunnel gre key 100
Switch(config-if)# ip address 192.192.168.1/24
Switch(config-if)# keepalive 5 3
Switch(config-if)# exit

Tunnel interface configuration for Switch2:

Switch(config)# interface tunnel1
Switch(config-if)# tunnel mode gre
Switch(config-if)# tunnel source eth-0-1
Switch(config-if)# tunnel destination 192.168.10.1
Switch(config-if)# tunnel gre key 100
Switch(config-if)# ip address 192.192.168.2/24
Switch(config-if)# keepalive 5 3
Switch(config-if)# exit

step 4 Configure the static route and arp

Configuring Switch1:

Switch(config)# ip route 192.168.20.0/24 192.168.10.2
Switch(config)# ip route 192.168.12.0/24 tunnel1

Configuring Switch2:

Switch(config)# ip route 192.168.10.0/24 192.168.20.2
Switch(config)# ip route 192.168.11.0/24 tunnel1

step 5 Exit the configure mode

Switch(config)# end

step 6 Validation

Display the result on Switch1:

Switch# show interface tunnel1
Interface tunnel1
Interface current state: UP
Hardware is Tunnel
Index 8193 , Metric 1 , Encapsulation TUNNEL
VRF binding: not bound
Internet primary address:
192.192.168.1/24 pointopoint 192.192.168.255
Tunnel protocol/transport GRE/IP, Status Valid
Tunnel source 192.168.10.1(eth-0-1), destination 192.168.20.1
Tunnel DSCP inherit, Tunnel TTL 255
Tunnel GRE key enable: 100
Tunnel GRE keepalive enable, Send period: 5, Retry times: 3
0 packets input, 0 bytes
0 packets output, 0 bytes

Display the result on Switch2:

Switch# show interface tunnel1
Interface tunnel1
Interface current state: UP
Hardware is Tunnel
Index 8193 , Metric 1 , Encapsulation TUNNEL
VRF binding: not bound
Internet primary address:
192.192.168.2/24 pointopoint 192.192.168.255
Tunnel protocol/transport GRE/IP, Status Valid
Tunnel source 192.168.20.1(eth-0-1), destination 192.168.10.1
Tunnel DSCP inherit, Tunnel TTL 255
Tunnel GRE key enable: 100
Tunnel GRE keepalive enable, Send period: 5, Retry times: 3
0 packets input, 0 bytes
0 packets output, 0 bytes