Vlan – Wake on LAN across VLANs in IPv6 network

Mystery solved.

The Windows 7 computer was continuously receiving TCP SYN packets to the 9100 port from another computer in the same LAN, waking up as a result when "Wake on Pattern Match" was activated.

This port is used by network printers. The IP of the Windows 7 computer belonged before to a long-time discarded HP printer.

The computer sending the packets is old, and it is always on as it is used sometimes to run software that only works in Windows XP. This computer still had the old network printer configured, pointing to the local IP address that now belongs to the Windows 7 computer having the problems. There were still documents in the printing queue, so the Windows XP computer was continuously sending TCP SYN packets to the Windows 7 computer, thinking it was the network printer. Removing the old printer from Windows XP and its associated TCP/IP Printer Port solved the problem.

I could not catch this traffic between both computers from mine using Wireshark. I had to install it on the affected computer and run it with promiscuous mode disabled.

Switches without MLD or IGMP snooping will treat multicast and broadcast (if there were broadcast in IPv6) the same -- namely, they would flood the packets out all ports.

But the second half of your statement,

"and therefore negate the benefits of multicast"

doesn't follow. The advantages of multicast are not for the switch. They are for the end hosts.

End hosts can process multicast more efficiently than broadcast packets. The network interface card (NIC) on most hosts can be set to accept specific multicast addresses and reject all others, just as they do with unicast packets. So unwanted multicast traffic is ignored by the NIC as well as unwanted unicast traffic. But when the host receives a broadcast packet, it must accept it and send it to the CPU to process, even if it is not important to this host. In a network with lots of broadcast traffic, hosts are continually interrupted to process the broadcasts, even though they care about very few of them.

Multicast solves this problem by "delegating" the task of selecting important packets to the NIC hardware. Only packets directly addressed to the host are sent to the CPU for processing.