macOS 15.6.1 - Thunderbolt bridge issue

Question

Level 1

4 points

macOS 15.6.1 - Thunderbolt bridge issue

Device 1: Studio Ultra M1, device 2: Studio Ultra M3. Both running macOS 15.6.1. The Ultras are connected

with a 0.8 m Thunderbolt 5 cable. On both Ultras, service order is set to 1) Thunderbolt Bridge, 2) Ethernet, 3) Wifi. In System Settings / Network, Thunderbolt Bridge is shown with "Self-assigned IP" on both Ultras. However, the Thunderbolt Bridge connection is NEVER activate for file transfers between the two Ultras. I have rebooted both devices, tested several Thunderbolt ports on each Ultra - no success.

Mac Studio, macOS 15.6

Posted on Aug 21, 2025 11:14 PM

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Answer 1

Aug 28, 2025 9:48 AM in response to rzbs

Every interface on every networked computer gives itself a pseudo-random IP address from the range of Self- Assigned IP addresses at startup. This is used to uniquely identify it on the local network

For devices connected to a Router serving DHCP addresses, that self-assigned IP address is replaced so fast that most users would swear the Self-Assigned address was never there.

But for local File access, a self-assigned IP address should be a perfectly good IP address.

Punchline:

Self-assigned IP address is what is expected for this link, unless you want to add Manual IP addresses yourself.

Now that I think about it, Manually assigning an IP address in the same range as your other IP addresses might be worth a try. Just pick ones outside of the DHCP addresses typically used but in the same subnet ( i.e., all octets match except that last 8 bits worth.) Give each device a UNIQUE IP address, and NOT the last one in the range xxx.yyy.zzz.255 (because that one is magical).

If any of that is just "word salad" post back and I or other readers can elaborate.

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Answer 2

WheelieNick

Level 8

45,805 points

Aug 22, 2025 3:11 PM in response to rzbs

Was this working before the update? Then it could be a glitch and you might want to report this to apple as feedback so that they can look into it

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Answer 3

rzbs Author

Level 1

4 points

Aug 23, 2025 12:27 AM in response to WheelieNick

Thanks - it was an issue that existed before the last update. In fact, the thunderbolt bridge connection has been unreliable since I started using it in April 2025. Sometimes the connection worked after both Ultras had been restarted, sometimes it didn't work at all. I have never been able to identify a specific pattern in the system settings that defined whether it worked or did not. I deactivated all network filters, e.g. Little Snitch, tested various network settings, cables, thunderbolt ports etc. - to no avail. Thunderbolt bridge fails or works "arbitrarily".

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