From ImageStream Router Documentation
Aggregate Ethernet Monitoring Setup
Hardware setup uses an Ethernet tap and requires 2 Ethernet ports on the router per Ethernet port tapped to monitor traffic in both directions. Our example uses Ethernet1 and Ethernet2 to receive the traffic from the tap. The ports are bridged together to allow a capture program to monitor both flows of traffic simultaneously.
interface bvi1 description Ethernet Ports 1+2 no ip address ! interface Ethernet1 no ip address bridge-group 1 spanning-disabled horizon 1 ! interface Ethernet2 no ip address bridge-group 1 spanning-disabled horizon 1 !
Even though the hardware tap disconnects the transmit lines we shouldn't be transmitting any data on the ports. We use the split-horizon feature of bridging to prevent packets received on one port from being transmitted on the other port.