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em—Management and internal Ethernet interfaces interfaces for TX Matrix Plus routers and T1600 ( and vQFX )
em0, which is an out-of-band management interface
em1, internal Ethernet interface which connects the Routing Engine
em2, on each Routing Engine (re0 and re1) in order to support fault tolerance.
fxp—Management and internal Ethernet interfaces for J Series, M Series, and MX Series routers