The stock caught a big downgrade on an unexpected competitive threat from Nvidia.Shares of data center switch juggernaut Arista Networks (ANET -8.61%) plunged on Friday, down about 8.6% as of 2:20 p.m. ET.While the entire tech sector and frankly the entire market was down today, Arista was doubly affected by an analyst downgrade. The content of the downgrade was especially worrying, given that Arista may find itself with a tough brand-new competitor: Nvidia.Nvidia is getting into Ethernet switchesAnalyst Mi ...