Here is something that looks wrong in the RB260 (CSS106) documentation.
This "manual" section shows this screen (not correct for v2.18)
Here's what my CSS106 (with SwOS v2.18) RSTP screen looks like: (red box added by me)
The text of the manual does describe the Forward Reserved Multicast functionality.
I haven't tested it, but if it does what it says it does, then it may fix @chechito's complaint here RSTP not working properly on CSS106-5G-1S - #4 by chechito but it does not seem to allow setting this on a port by port basis.
Here's what the text says as of 2026-02-16
The reason I was looking at this was because I just switched to ATT Fiber 300 (from Spectrum cable) and wanted to put a tap to watch traffic on the "WAN" side of the router (between my home router and the ATT BGW320 LAN) but I don't want RSTP or LLDP stuff being transmitted from the switch ports connected to the ISP side. I have the ATT BGW320-505 in passthrough mode, but that's a "hack" which evidently masquerades all traffic to the BGW320's LAN, and then reapplies source NAT to make it appear to the home router as if it is the global ip address, and bypasses the BGW320's firewall.