I am running EdgeRouter.ER-e100.v1.8.0.4853089.160219.1607, and seeing an awfully large amount of traffic transmitted on my eth1 to the LAN side of my network, completely unrelated to the traffic transmitted on eth0 (WAN), and received by both interfaces.
Uptime:
22:56:32 up 67 days, 3:45, 1 user, load average: 0.89, 0.67, 0.50
eth0:
Transmitted: 1.33 TB
Received: 1.44 TB
eth1:
Transmitted: 282.91 TB
Received: 1.34 TB