Visimation is Cisco's Visio developer and has been producing Cisco Systems Visio Stencils since 2008:
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Cisco posts its Visio stencils on the Cisco Visio Stencils download page
Cisco Visio Stencil User Guide and FAQ
Cisco also produces network toplogy icons internally and with several graphics design firms including Visimation. Most of these icons have been produced by others