Which layer is the problem: the carrier's transport, or your own side?

When a network service degrades, the fault sits in one of two places: the carrier's Layer 2 transport, or the customer's own Layer 3 side. Measuring both layers on the same circuit, at the same time, shows which, because the layer that fails is the layer at fault.

Why “whose problem is it” is so hard to answer

Most degradations turn into an argument. The customer sees slow applications; the carrier sees a healthy circuit. Each is looking at different data, or no shared data, so the first hour of every incident goes on attribution rather than resolution, and the service stays degraded while it does.

What each layer tells you

Layer 3 monitoring measures IP reachability and performance, whether traffic gets through, and how well, typically from independent vantages. Layer 2 monitoring measures the carrier Ethernet transport beneath it: frame loss, frame delay and delay variation, on the line itself. One is the road surface; the other is whether the journey completed.

Reading the two layers together

Run both on one circuit and the combination is the diagnosis:

• Layer 2 down and Layer 3 down: a carrier transport fault, confirmed at two layers.
• Layer 2 healthy but Layer 3 degraded: the transport is fine, the issue is on the customer or IP side.
• Layer 2 healthy and only one Layer 3 vantage affected: an internet path, not the service.

The answer is shown on the circuit, not argued over a call.

Why attribution is worth the measurement

Every hour not spent proving whose fault it is, is an hour towards fixing it. And attribution backed by independent measurement changes the conversation with a provider: a claim they can’t wave away, because it isn’t their own number.

How XVOLV does it

Parallax measures Layer 2 carrier Ethernet on the line alongside its independent Layer 3 vantages, on the same circuit, and records the result in a deterministic evidence pack. When both layers run, each incident shows which layer it sits on. Run either layer on its own, or both together. This is layer-aware proof: a source-agnostic SLA engine that computes the service level the same deterministic way whatever layer the measurement comes from, with which-layer attribution when both are read together. The reasoning behind the method is set out in the white papers The Accountable Line and Proof by Rule.

Frequently asked questions

Can I tell if it's the carrier or me without their cooperation?

Yes. Measuring both layers on the circuit does not depend on the carrier’s systems or sign off. Independent Layer 3 vantages test reachability from outside, and Layer 2 is measured on the line, so the attribution rests on your own evidence rather than their report.

What Layer 2 metrics are measured?

The carrier Ethernet performance metrics: frame loss ratio, frame delay and frame delay variation, following the MEF framework and measured with ITU-T Y.1731 and IEEE 802.1ag mechanisms.

Does this replace my existing monitoring?

No. It sits alongside it. Most monitoring watches a single layer; reading Layer 2 and Layer 3 together on the same circuit adds the attribution that single layer monitoring cannot give on its own: whose fault it is.

How fast does it attribute a fault?

As fast as both layers are being measured. When Layer 2 and Layer 3 run continuously on the circuit, each incident already carries the two readings needed to place it, so attribution is available at the moment of the incident rather than after an investigation.

Sources

• MEF, carrier Ethernet service and SLA definitions. mef.net
• ITU-T Y.1731, OAM functions and mechanisms for Ethernet-based networks. itu.int
• IEEE 802.1ag, Connectivity Fault Management. standards.ieee.org