Technical skills depend on the team you are placed into but overall good programming and analytical skills. In addition, you should be flexible and willing to learn any technology the team is working on.
Apart from that I would say networking skills might come handy since there are plenty of intern events during the internship period and 2-3 outings/ picnics/ lunches are arranged for the interns, so a lot of opportunities to connect with people.
Even for the work you are assigned during the internship, sometime you have to contact other teams as well for information, so be upfront in asking questions, not just your team but anyone in Cisco. People are super helpful and usually reply to any of the queries you have. Infact in the orientation only, the coordinators will be encouraging you to ask questions/ doubts to anyone in Cisco without any hesitation.