Big news today for networking startup Plexxi, which announced it had secured $20.1 million in Series C funding from existing investors.

The $20 million comes from North Bridge Partners and Matrix Partners, both based in the Greater Boston area, as well as Silicon Valley-based Lightspeed Venture Partners.

In July of 2011, Plexxi had raised a total of $28.38 million from these three venture firms during the first two rounds of funding. The Series C investments bring their grand total to just north of $48 million.

Plexxi is a network solution that takes the needs of their clients into account and adapts their network to those needs in real-time. For example if a client needs more bandwidth, they get it; if they need an added layer of security, boom it’s there.

With $48 million under its belt, Plexxi is looking to hit the ground running with their private beta tests that were initially launched on June 19. Already 30 employees strong, Plexxi looks to add another 15 by the end of the year for both their Cambridge office and their Nashua, NH office.

We featured Plexxi in an April article profiling 62 companies to keep an eye out for, and it looks like they won’t disappoint. Expect their beta testing to be in full swing by July though the software won’t be commercially available until the end of the year or early 2013.