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I switched to Deutsche Telekom Glasfaser (FTTH) and wanted to keep my MikroTik router. No FritzBox, no rented hardware, just my own box talking to the ONT.

It took a couple of hours to get it working, and the journey involved a fair amount of trial and error. Here’s what I tried, what failed, and what finally worked.

The setup 🔗

┌─────────┐         ┌─────┐          ┌──────────┐
│  Fiber  │──fiber──│ ONT │──eth────▶│ MikroTik │
└─────────┘         └─────┘          │  ether1  │
                                     └──────────┘

What I tried (and failed) 🔗

Attempt 0: swap the modem 🔗

My old setup was straightforward: a DSL modem on ether1 (the WAN port), with the MikroTik running a DHCP client to get an IP from it.

I unplugged the DSL modem, plugged in the fiber ONT on the same port, and kept the same config. Nothing happened. DHCP stayed searching.

Attempt 1: DHCP on bare ether1 🔗

Same idea, but with a fresh DHCP client on the WAN port.

/ip dhcp-client add interface=ether1 add-default-route=yes use-peer-dns=yes

DHCP stayed in “searching” forever. A packet capture showed the discovery going out, but no reply ever came back.

Attempt 2: DHCP on VLAN 7 🔗

After some digging, I found that Telekom uses VLAN 7 for internet traffic. This isn’t documented on Telekom’s official setup page; the only sources are community forum posts, third-party guides, and ISP parameter lists floating around German networking forums.

I tagged the port and tried again:

/interface vlan add interface=ether1 name=vlan7 vlan-id=7
/ip dhcp-client add interface=vlan7 add-default-route=yes use-peer-dns=yes

Same result. No reply.

Attempt 3: static IP through the ONT 🔗

The ONT has a management interface at 192.168.100.1. Setting a static IP in that subnet worked for reaching the ONT itself:

/ip address add address=192.168.100.2/24 interface=ether1
/ping 192.168.100.1  # works
/ping 8.8.8.8        # nothing

The ONT was reachable, but the internet was not. It simply wasn’t forwarding traffic.

Attempt 4: proving it’s not the router 🔗

Telekom support wasn’t particularly helpful. They said they “don’t support exotic routers” and asked whether my router “supports WAN.” No technical details, no real troubleshooting.

So I decided to prove it myself. I plugged a laptop directly into the ONT and ran the same tests:

sudo ip addr add 192.168.100.2/24 dev eno1
sudo ip route add default via 192.168.100.1
ping 192.168.100.1  # works
ping 8.8.8.8        # nothing

Same behavior, no router involved. The ONT itself wasn’t passing traffic to the internet.

What worked: PPPoE on VLAN 7 🔗

Finally, after more searching and getting some help from colleagues, I tried PPPoE on VLAN 7:

/interface vlan add interface=ether1 name=vlan7 vlan-id=7
/interface pppoe-client add interface=vlan7 name=pppoe-telekom user=test password=test disabled=no

It connected. A public IP was assigned and I had internet.

Interestingly, the PPPoE credentials didn’t even matter: user=test password=test was enough. The ONT handles authentication via PLOAM (the fiber-level protocol), so Telekom doesn’t actually check PPPoE credentials on this line type. PPPoE is just the mechanism for IP assignment.

Full working config 🔗

Internet (PPPoE on VLAN 7) 🔗

/interface vlan add interface=ether1 name=vlan7 vlan-id=7
/interface pppoe-client add interface=vlan7 name=pppoe-telekom user=test password=test disabled=no
/ip firewall nat add chain=srcnat out-interface=pppoe-telekom action=masquerade

ONT web UI access from LAN 🔗

The ONT has a management page at 192.168.100.1. To reach it from LAN devices, the router needs an IP in the same subnet.

/ip address add address=192.168.100.2/24 interface=ether1

This works because ether1 carries two types of traffic side by side:

Since ether1 is in MikroTik’s WAN interface list, the default masquerade rule handles NAT for LAN → ONT traffic. No extra rules are needed.

Why VLAN 7 and PPPoE 🔗

Telekom uses VLAN tags to separate services on the same physical wire. VLAN 7 is for internet traffic. Untagged frames get ignored, which is why bare DHCP didn’t work.

PPPoE (Point-to-Point Protocol over Ethernet) wraps the traffic in a point-to-point tunnel. Telekom’s server (BRAS/BNG) assigns a public IP through this tunnel.

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