ARP sponge is an application that snoops on ARP packets on the Virtual Private LAN Service (VPLS) domain.
ARP sponge listens for ARP traffic. When the number of ARP requests for a certain IP address exceeds a threshold, ARP sponge sends out an ARP reply for that address that uses its own MAC address. This achieved by using the insight LAG to the bridge domain as an AC endpoint. All ARP traffic received by the VPLS instance is flooded to the insight LAG as well.