I've purchased 7 CSS326-24G-2S+ with 7 SFP+ 1m direct attach cable, for mount a ring using RSTP.

Connecting on one unit, and doing PING to all units, all replies to ICMP except one unit that loses some time, some time not. Breaking the loop, it works as expected.

Tried a small ring with only 3 switch's, with the same result.

All units up to date with firmware 2.7. All with RSTP enabled on all ports.