The ISC blocking filters are used to prevent looping and optimize bandwidth utilization.
When at least one MLAG peer port is active, the upper layer software initiates a block of traffic that ingresses the ISC port and needs to be forwarded to the local MLAG ports. This is considered to be the steady state condition. In normal steady state operation most network traffic does not traverse the ISC. All unicast packets destined to MLAG ports are sent to the local MLAG port only. However, flood and multicast traffic will traverse the ISC but will be dropped from MLAG peer port transmission by the ISC blocking filter mechanism. The ISC blocking filter matches all Layer 2 traffic received on the ISC and blocks transmission to all MLAG ports that have MLAG peer ports in the active state.
When there are no active MLAG peer ports, the upper layer software initiates an unblocking of traffic that ingresses the ISC port and needs to be forwarded to the local MLAG ports thus providing redundancy. This is considered to be the failed state.