This read-only OCTET STRING is an 8-octet bitmap identifying which incoming port(s) this multicast origin route was learned on, with the least significant bit of the eighth octet representing port 0, continuing up to a maximum of 64 ports. An admin decodes this bitmap to determine on which physical or logical interfaces the device has learned reachability to a given multicast source. Decoding sipmRouteParents to find multiple bits set would tell the admin the device has learned this origin route via more than one incoming interface, useful for understanding multicast topology redundancy.