S5700-x-li can not stack with S5720-x-li.

NOTE:
* Any models of the S5700S-X-LI series can set up a stack, but S5700S-X-LIs cannot set up a stack with S5700S-P-LIs. (for example S5700-28X-LI-AC and S5700-52X-LI-AC can stack together, must use 10gig SPF+)

* Any models of the S5720-LI series can set up a stack. When different models need to set up a stack, they must use stack ports of the same type. That is, they all use electrical ports on front panels or SFP/SFP+ optical ports. (for example, S5720-28P-LI-AC and S5720-28X-LI-AC can stack together. you can use 1 gig copper, 1gig SPF, or 10gig SPF+)