[Day 12] 4. GRE Tunnel Configuration

Network Topology Overview

Router A (LAN Side): Network
```
172.16.10.0/24
```
- Router A g0/1 (Public/Internet Interface): IP
```
75.1.1.1
```
- Router A Tunnel0 Interface: IP
```
172.16.20.1/24
```
Router B (LAN Side): Network
```
172.16.30.0/24
```
- Router B g0/1 (Public/Internet Interface): IP
```
80.1.1.1
```
- Router B Tunnel0 Interface: IP
```
172.16.20.2/24
```
Tunnel Between A and B:
- Tunnel0 IP on A:
```
172.16.20.1/24
```
- Tunnel0 IP on B:
```
172.16.20.2/24
```

Purpose

You are configuring a GRE tunnel between Router A and Router B to enable communication between their respective LAN networks:

Router A's LAN:
```
172.16.10.0/24
```
Router B's LAN:
```
172.16.30.0/24
```

The GRE tunnel allows traffic between these private LANs to traverse the internet using encapsulation.

Configuration Breakdown

Router A Configuration

A(config)# ip route 172.16.30.0 255.255.255.0 172.16.20.2

Static route: Configures a route to reach Router B's LAN network (
```
172.16.30.0/24
```
) via the tunnel's destination IP (
```
172.16.20.2
```
), which is the Tunnel0 IP of Router B.

A(config)# int tunnel0

A(config-if)# no shut

A(config-if)# ip address 172.16.20.1 255.255.255.0

A(config-if)# tunnel source 75.1.1.1

A(config-if)# tunnel destination 80.1.1.1

A(config-if)#ip mtu 1420

A(config-if)# exit

Tunnel Interface (
```
tunnel0
```
):
- IP address: Configured as
```
172.16.20.1/24
```
  (the local end of the tunnel).
- Tunnel source: The public IP of Router A's interface (
```
75.1.1.1
```
  ), facing the internet.
- Tunnel destination: The public IP of Router B (
```
80.1.1.1
```
  ), which the tunnel will terminate at.
- MTU (Maximum Transmission Unit): Set to
```
1420
```
  , which allows proper packet sizing and avoids fragmentation issues within the tunnel.

Router B Configuration:

B(config)# ip route 172.16.10.0 255.255.255.0 172.16.20.1

Static route: Configures a route to reach Router A's LAN network (
```
172.16.10.0/24
```
) via the tunnel's destination IP (
```
172.16.20.1
```
), which is the Tunnel0 IP of Router A.

B(config)# int tunnel0

B(config-if)# no shut

B(config-if)# ip address 172.16.20.2 255.255.255.0

B(config-if)# tunnel source 80.1.1.1

B(config-if)# tunnel destination 75.1.1.1

B(config-if)#ip mtu 1420

B(config-if)# exit

Tunnel Interface (
```
tunnel0
```
):
- IP address: Configured as
```
172.16.20.2/24
```
  (the local end of the tunnel on Router B).
- Tunnel source: The public IP of Router B's interface (
```
80.1.1.1
```
  ), facing the internet.
- Tunnel destination: The public IP of Router A (
```
75.1.1.1
```
  ), where the tunnel will terminate.
- MTU: Set to
```
1420
```
  , ensuring compatibility with the tunnel MTU on Router A and avoiding fragmentation.

How the GRE Tunnel Works ?

The GRE tunnel connects Router A and Router B across the internet, allowing the private networks on either side (
```
172.16.10.0/24
```
and
```
172.16.30.0/24
```
) to communicate as if they were on the same local network.
The static routes on both routers ensure that traffic intended for the other LAN (e.g., traffic from
```
172.16.10.0/24
```
destined for
```
172.16.30.0/24
```
) is forwarded through the tunnel.

Key Points

The tunnel encapsulates private traffic from the LANs, allowing it to travel across the public internet without being exposed.
MTU of
```
1420
```
is specified to ensure that packets fit within the maximum transmission size across the tunnel without causing fragmentation.
The static routes on both routers direct LAN traffic into the tunnel for cross-network communication.

This configuration effectively creates a secure, private link between the two routers and their LANs over the public internet using GRE encapsulation.