Within the encap tunnel group named in the notification, this identifies which specific member tunnel is currently the active one carrying traffic, as opposed to a standby/protection tunnel in the same group. This is operationally significant because encap groups are built for path protection, so tracking which index is active tells the admin whether a protection switchover has just occurred. If the active index changes from 3 to 8 between two notifications, that's evidence the group just failed over to its backup tunnel.