blog_tech/i18n/en/docusaurus-plugin-content-docs/current/openwrt/gretap-vlan.md

---
sidebar_position: 3
title: GREtap Tunnels for VLANs
---

# GREtap Tunnels for VLANs Across the Mesh

This guide explains how to extend VLANs across the WiFi mesh backhaul using GREtap tunnels.

## Prerequisites

- WiFi mesh backhaul configured according to the [previous guide](./backhaul-wifi-mesh.md)
- Both routers must be able to communicate via the mesh interface
- Access to the LuCI interface on both routers

## Why GREtap Instead of BATMAN-adv?

For a mesh network consisting of **only two routers**, GREtap is preferable to BATMAN-adv for several reasons:

### Advantages of GREtap in This Scenario

1. **Point-to-Point Sufficient**: With two routers, a simple point-to-point tunnel is sufficient. BATMAN-adv is designed for complex mesh topologies with many nodes and redundant paths.

2. **Less Overhead**: GREtap has lower protocol overhead than BATMAN-adv. With only two nodes, the advanced mesh routing features of BATMAN are not necessary.

3. **Configuration Simplicity**: GREtap is simpler to configure and debug. No need to manage mesh routing tables, path metrics, or route selection algorithms.

4. **Predictability**: Traffic always takes the same path (the direct tunnel). No dynamic route changes.

5. **Native Support**: GREtap is widely supported and documented in OpenWRT without requiring complex additional kernel modules.

:::info BATMAN-adv: When to Use It?
BATMAN-adv becomes interesting when you have:
- 3 or more mesh nodes
- Multiple possible paths between nodes
- Need for redundancy and automatic failover
- Dynamic mesh topology with mobile nodes
:::

## Installing Required Packages

Install the necessary package on **both routers**:

```bash
opkg update
opkg install luci-proto-gre
```

Or via the LuCI interface: **System → Software**, search for and install `luci-proto-gre`.

## Configuring the GREtap Tunnel

### Step 1: Create the GREtap Interface

On **both routers**, create a new GREtap interface:

1. Go to **Network → Interfaces → Add new interface**
2. Configure the basic parameters

**Basic Parameters**:
- **Name**: short name (example: `gr`)
- **Protocol**: GRETAP (Ethernet over GRE)

:::danger Interface Name Length Limitation
OpenWRT automatically creates the interface with the `gretap-` prefix. For example, if you name your interface `trunk`, OpenWRT will create `gretap-trunk`.

**Problem**: To pass VLANs, the notation will be `gretap-trunk.100`, which is **16 characters** and exceeds the limit!

**Problematic Example**:
- Interface named `trunk` → Device created: `gretap-trunk` (13 characters)
- VLAN 100 → `gretap-trunk.100` (16 characters) ❌ **TOO LONG**

**Solution**: Use a **very short** name like `gr`, `t`, or `g`.
- Interface named `gr` → Device created: `gretap-gr` (9 characters)
- VLAN 100 → `gretap-gr.100` (13 characters) ✅ **OK**

:::

:::info Origin of This Limitation
Network interface names under Linux are stored in a structure that uses a 16-byte array. This array includes the null terminator `\0`, so the maximum length of a network interface name is **15 characters** (16 - 1 = 15).
:::

**Tunnel Parameters (General Settings)**:
- **Remote IPv4 address or FQDN**: IP of the other router on the mesh interface
- **Local IPv4 address**: IP of this router on the mesh interface

![GREtap interface configuration](/img/openwrt/interface-gretap.png)

**Advanced Options (Advanced Settings)**:

It is important to configure the advanced options correctly:

- ⬜ **Use TTL on tunnel interface**: leave unchecked
- ⬜ **Use PMTU discovery**: leave unchecked (equivalent to **Don't fragment**)
- ⬜ **Default gateway**: UNCHECK this option (important!)
- **Bind interface**: select the mesh interface (for example `lan`)

![GREtap advanced parameters configuration](/img/openwrt/interface_gretap_parametresavance.png)

:::tip Why Uncheck These Options?
- **PMTU discovery / Don't fragment**: WiFi mesh traffic may require fragmentation. Allowing fragmentation avoids packet loss.
- **Default gateway**: The GREtap tunnel should not become the router's default gateway.
:::

## Extending VLANs Through the Tunnel

### Principle

To pass a VLAN through the GREtap tunnel, you need to create a **bridge device** that contains:
- The local physical port or VLAN interface
- The corresponding GREtap VLAN port

The notation used for a GREtap VLAN port is: `@<gretap_device_name>.<vlan_number>`

**Example**: For VLAN 100 with a tunnel named `gr` (device: `gretap-gr`) → `@gretap-gr.100`

### Configuration on the Main Router

#### Step 1: Create the Bridge Device for the VLAN

1. Go to **Network → Interfaces → Devices → Add device configuration**
2. Create a **Bridge device**:

**Bridge Configuration**:
- **Device type**: Bridge device
- **Device name**: `br-lab` (or descriptive name for your VLAN)
- **Bridge ports**: Add the following ports:
  - The local physical port or VLAN (example: `lan3` for a physical port)
  - The GREtap VLAN port: `@gretap-gr.100` (adapt according to your device and VLAN number)

![Bridge device with GREtap port](/img/openwrt/bridge_device_lab_avec_port_gretap.png)

#### Step 2: Create the Network Interface for the VLAN

1. **Network → Interfaces → Add new interface**
2. Configure the interface:

**Parameters (General Settings)**:
- **Name**: `LAB` (or descriptive name)
- **Protocol**: Static address
- **Device**: Select the previously created bridge (`br-lab`)
- **IPv4 address**: This router's IP address on the VLAN (example: `192.168.100.1`)
- **IPv4 netmask**: The VLAN's netmask (example: `255.255.255.0`)

![VLAN interface with bridge as device](/img/openwrt/interface_vlan_lab_avec_bridge_en_device.png)

**DHCP Server (DHCP Server)**:

Configure the DHCP server for this VLAN to distribute IPs to clients:

![VLAN DHCP configuration](/img/openwrt/interface_vlan_lab_dhcp_sur_routeur_maitre.png)

**Firewall Settings**:

Assign the interface to an appropriate firewall zone (example: `homelab_zone` or create a new zone):

![Firewall zone configuration](/img/openwrt/interface_vlan_lab_firewallzone.png)

:::tip Static DHCP Leases
Once the Dumb AP retrieves an IP via DHCP on this VLAN, you can configure a **static DHCP lease** to assign it a fixed IP.

Example: set the Dumb AP to IP `.2` on each VLAN for consistent configuration (example: `192.168.100.2`).
:::

Repeat this configuration for each VLAN to extend through the tunnel.

### Configuration on the Dumb AP

On the second router (Dumb AP), the VLAN interfaces must be configured as **DHCP clients** to automatically retrieve an IP on each VLAN.

1. **Network → Interfaces → Add new interface**
2. Configure the interface:
   - **Name**: `vlan100` (or descriptive name)
   - **Protocol**: DHCP client
   - **Device**: `@gretap-gr.100` (the GREtap VLAN port)

The Dumb AP will automatically retrieve an IP address from the main router's DHCP server on this VLAN.

### Multi-VLAN Configuration Example

**Typical Configuration**:
- VLAN 10 (Management) → `@gretap-gr.10`
- VLAN 100 (Homelab) → `@gretap-gr.100`
- VLAN 200 (IoT) → `@gretap-gr.200`

Each VLAN crosses the GREtap tunnel transparently and in isolation.

## Verification and Testing

### Connectivity Tests

From the Dumb AP, test connectivity on each VLAN:

```bash
# Ping to VLAN 100 gateway
ping -I vlan100 192.168.100.1

# Verify obtaining an IP via DHCP
ip addr show dev gretap-gr.100
```

From the main router, verify that the Dumb AP appears in the DHCP leases:

- **Network → Interfaces → LAB → DHCP Server → Active DHCP Leases**

## Conclusion

With GREtap, you can effectively extend your VLANs across a WiFi mesh backhaul between two OpenWRT routers. This solution offers a good compromise between simplicity, performance, and functionality for point-to-point topologies.

For more complex mesh networks with 3 or more nodes, consider using BATMAN-adv which offers more advanced mesh routing features.