To configure VLANs with **static IP addresses** on Debian using the native `/etc/network/interfaces` file, follow this setup. This replicates your current configuration using VLANs on `eth0` and assigns static IPs to `eth0.20` and `eth0.30`.

---

### 📁 **/etc/network/interfaces Configuration**

```bash
# Define VLAN interfaces with static IPs
auto eth0
iface eth0 inet static
    address 10.56.0.243/24
    gateway 10.56.0.1
    dns-nameservers 8.8.8.8 8.8.4.4

auto eth0.20
iface eth0.20 inet static
    address 10.56.2.103/24
    gateway 10.56.2.1
    dns-nameservers 8.8.8.8 8.8.4.4
    inet6 address 2001:db8::1/64
    inet6 gateway 2001:db8::1
    inet6 dns-nameservers 2001:4860:4860::8888 2001:4860:4860::8844

auto eth0.30
iface eth0.30 inet static
    address 10.56.3.108/24
    gateway 10.56.3.1
    dns-nameservers 8.8.8.8 8.8.4.4
    inet6 address 2001:db8::2/64
    inet6 gateway 2001:db8::2
    inet6 dns-nameservers 2001:4860:4860::8888 2001:4860:4860::8844
```

---

### 🧰 **Steps to Apply the Configuration**

1. **Install `vconfig`** (required for VLANs):
   ```bash
   sudo apt update
   sudo apt install vconfig
   ```

2. **Create VLAN interfaces** (if not already created):
   ```bash
   sudo vconfig add eth0 20
   sudo vconfig add eth0 30
   ```

3. **Update `/etc/network/interfaces`** with the above configuration.

4. **Restart networking**:
   ```bash
   sudo systemctl restart networking
   ```

5. **Verify interfaces**:
   ```bash
   ip a
   ```
   You should see `eth0.20` and `eth0.30` with static IPs assigned.

---

### 📌 **Key Notes**

1. **VLAN Interface Creation**:
   - The `vconfig` tool creates the VLAN interfaces (`eth0.20`, `eth0.30`).
   - Ensure `vconfig` is installed to manage VLANs.

2. **Static IPs**:
   - Use `address` for IPv4 and `inet6 address` for IPv6.
   - Set `gateway` and `dns-nameservers` for routing and DNS.

3. **IPv6 Support**:
   - IPv6 addresses are configured using `inet6 address` and `inet6 gateway`.
   - Ensure your network supports IPv6 or adjust accordingly.

4. **Parent Interface (`eth0`)**:
   - `eth0` is configured with a static IP (`10.56.0.243/24`) and a default gateway (`10.56.0.1`).

---

### 🧪 **Validation**

After applying the configuration:
- Check IP addresses:
  ```bash
  ip a
  ```
- Test connectivity:
  ```bash
  ping -c 4 8.8.8.8
  ping -c 4 2001:4860:4860::8888
  ```

---

### 📝 Summary

This `/etc/network/interfaces` configuration sets up VLANs (`eth0.20`, `eth0.30`) with **static IPv4 and IPv6 addresses**, matching your original setup. It avoids manual `ip link` commands and ensures consistent networking behavior.