ERSPAN enables monitoring of traffic at multiple source ports and then transmitting this traffic over Layer 3 network to a specific destination port. Multiple source ports can be mirrored to a single destination port. However, you cannot mirror a source port to multiple destination ports.
ERSPAN uses Generic Routing Encapsulation (GRE) and thus, is routable across the network.
You use the monitor session command to enable an ERSPAN session. With this command, you can set the packet source, the packet destination (using the erspan key of this command), and the packet direction (egress, ingress, or both).