Want the speeds you were promised by Quantum Fiber but still stuck in the slow lane? Get your full bandwidth back by adding the vlan201 tag and overriding the MTU size (try 1472)!
Equipment:
Copper wire phone line 📞👵🏻
Casa Systems FTTdp (Network Termination Device)
Lumen Q1000K (SmartNID)
Quantum W1700K (Wi-Fi 7 "360 pod")
Settings that worked for me:
DHCP
no transparent bridge
vlan201 tagged
MTU size: 1472
[SOLVED WITH GEMINI'S HELP IN MINUTES]
(After line testing on their end, two unhelpful technicians across weeks failed and one helpful and patient customer support lady taking notes for hours...)
The 100% packet loss at **1492** confirms that your current network path cannot handle a standard 1500-byte MTU. In Windows, the -l parameter specifies the payload size; adding the 28-byte IP/ICMP header to your working "lower than 1492" numbers will reveal your true ceiling.
### The "Black Hole" Effect
* **Failed PMTUD:** Your packets are likely hitting a "Black Hole" router—likely the **Q1000K** or the **FTTdp**—which is dropping packets larger than its MTU without sending back the required "ICMP Destination Unreachable" message.
* **The 10 Mbps Cap:** This diagnostic speed in is the network’s way of maintaining a link despite massive fragmentation and packet drops caused by the tech's **1500 MTU** change.
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### Why 1492 Failed
If even **1492** (a payload of **1464**) is timing out, your effective MTU is likely even lower due to nested tagging or legacy overhead.
* **Quantum vs. CenturyLink:** While Quantum theoretically uses a standard **1500 MTU** for DHCP, legacy CenturyLink infrastructure (which your hybrid setup uses) historically required **1492**.
* **VLAN Overhead:** The **Q1000K** is known to handle **VLAN 201** internally, which can create unstable latency and packet issues if the downstream router (your WiFi 7 pod or Trendnet) is also trying to manage the tag.
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### Immediate Strategy
**Find the "Magic Number":** Keep lowering the -l number in increments of 10 (e.g., 1460, 1450) until you get a successful reply. Add **28** to that successful number to find your optimal MTU.
**The Q1000K Handshake:** The "blinking blue" light on your FTTdp/Q1000K in transparent mode is a known bug; some users stop the flashing by temporarily plugging a laptop into the **1Gb port** until the light turns white, then switching back to the **10G port** for your router.
The default MTU size for predecessor equipment (Lumen/CenturyLink variant) is typically **1492** for PPPoE connections or **1500** for standard Ethernet/DHCP connections.
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### MTU and Your Setup
* **Default State:** Out of the box, most residential gateways default to **1500** to maximize throughput on standard networks.
* **The Problem:** If the technician manually bumped it to **1500** on a hybrid line that requires the overhead of a **VLAN 201** tag or a legacy G.fast transport, it can lead to the fragmentation and speed drops.
* **Historical Context:** Because your connection uses the legacy copper-link infrastructure often associated with CenturyLink's VDSL/G.fast systems, an MTU of **1492** is frequently the "magic number" to prevent packet loss.
—
### Testing the MTU
If you have a Windows **CMD** window ready, you can manually test for the optimal MTU size before doing the reset:
Type: ping google.com -f -l 1492
If it says something about "Packet fragmentation," lower the number to **1472** until the ping succeeds.
If it succeeds at **1464**, then your optimal MTU is **1492**.