Thanks for support! For the record, the issue vanished some time after I asked the question and hasn't reappeared. I guess it was a hick-up caused by some earlier configuration change and it might be difficult to reproduce it.
We have several S4148 switches with OS version 10.5.0.0. The problem is that the switches are sending a constant stream of IPv6 neighbor solicitations querying the other nodes connected to the network, without any obvious reason. Sometimes it is one or a few requests / second, sometimes a burst of 30-100 messages. I don’t see any other switches (including OS9) doing the same. The network is IPv6 enabled, i.e. there is a router advertising the prefix and clients and servers using it. However, there is no IPv6 configuration on the switches other than the default configuration (“show running-config | grep ipv6” shows nothing). According to “show ipv6 interface brief” all VLANs have the same link local address enabled but no global addresses. The switches have IPv4 addresses for management but no other L3 functionality. “show ipv6 neighbors” shows a rather long list of addresses. Is there a reason why the switches are so interested about their neighbors with the default configuration?
OS10 switch flooding IPv6 neighbor solication requests