Hi!
All X440s (assuming they're not G2 generation) has dedicated stacking ports except the Summit X440-24t-10G and X440-24p-10G. Those two latter can be stacked with the 10 G ports in the front. The dedicated stacking ports are really 10 G ports, but marketing-wise they say it 40 G SummitStack, where they count each link both TX and RX of 10 G and then you have two links, so that makes it 10 G * 2 links * 2 directions (TX/RX), making it 40 G stacking... Well, they're not alone in doing so, but in reality, it's 10 G links.
For the original X450 (not X450e and X450a), you need to use SummitStack modules that you put in the rear of the switch.
SummitStack ports require a special cable, not the standard DAC or RJ45.
If you mean G2 models, stacking is done via 10 G SFP ports or, in some models, with a special 21 G port:
"Up to eight Summit X450-G2 switches can be stacked using two different methods of stacking: SummitStack-V, and SummitStack-V84. Each X450-G2 unit comes equipped with 2 ports of 21 Gigabit stacking ports via a QSFP interface. Standard passive 40Gb copper cables can be used for stacking X450-G2s together."
Hope this helps!
/Fredrik
PS Sorry, didn't read the subject line, but the answer is there for X450-G2 as well As mentioned, SummitStack-V is 4x10G more or less...
