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g/av • Posted by u/dante_dave •

Dante clock drift on a two-building campus fiber link — PTP over a routed segment Networked AV

Two buildings, single mode fiber between them, Dante audio distributed across both. About 60 devices total.

Symptom: devices in building B intermittently drop out of sync, Dante Controller shows clock status flapping between "Slave" and "Not synced." Building A is rock solid.

The fiber link between buildings is a routed L3 segment because the network team will not extend the VLAN. Everything else across that link is fine — control, video, management.

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u/cat6_carlos •
✓ Accepted Solution

The reply above is correct on the cause and there are two real fixes depending on what the network team will actually agree to.

Fix 1 — keep it layer 2. Ask for a point-to-point L2 pseudowire or a dedicated VLAN extended across the fiber for audio only. Network teams object to "extend a VLAN between buildings" as a general policy and often accept "one dedicated VLAN, no gateway, audio only" because there is no routing exposure. Frame it that way.

Fix 2 — give building B its own clock. Run a second Dante clock master in building B with both locked to an external reference. Word clock or GPS-disciplined. Then you are not asking PTP to cross the boundary at all, you are just asking two domains to agree on time via a source outside the network.

Fix 1 is cheaper and Fix 2 is more robust. On a campus that is going to keep growing, I would do Fix 2 and stop having this argument every time they add a building.

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✓ Solution
u/dante_dave OP •

Fix 2 is the answer for this site. They are adding a third building next year and I would rather solve it once. Pricing GPS-disciplined masters now.

The framing on Fix 1 is useful regardless — "one VLAN, no gateway" is a much smaller ask than what I have been making.

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u/av_wizard •

PTP across a routed boundary is the problem. Dante uses PTPv1 by default and it is designed for a single layer 2 domain. Once you route it, the multicast that carries the clock has to be handled by the router and most of them handle it badly or not at all.

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u/crestron_carl •

Worth noting the same problem hits NVX video over routed segments for the same reason, and people usually discover it separately about a year later. If you are putting in an external clock reference for audio, size the plan for video too.

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u/wireless_wendy •

Small thing from the network side — if you do end up going the L2 route, ask specifically whether their fiber link has any QoS policy at all. A lot of campus links are wide open and "everything else works fine" is true right up until the day somebody starts a backup job across it.

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