Kyle Okposo, Sundstrom and Justin Johnson gave the Islanders a 3-0 lead after the first period. Sundstrom added his second goal in the middle period and John Tavares scored in the third for the Islanders.
"With our 5-on-5 and special teams play, we've accomplished what we wanted to accomplish," Islanders coach Jack Capuano said. "We've still got Tuesday, Wednesday and Thursday of next week to work more on those things. All of our guys have battled for the last few weeks. Unfortunately, you can only keep so many of them."
Robin Lehner stopped 33 shots for the Senators.
Material from wire services was used in this report.