There are others who interpret it differently. Ignorance is defined as not knowing the difference between the body and the soul (self): Body-soul identification is ignorance. In reality they are two entities. The ego, the veiling wall of darkness, asserts mine-ness of the body and the world and conceals the self, that is the sun. The self is stable and immutable in its structure, functions and stance, while the ego's targets, aspirations, and desires change with time and age. Self's stability against changing desires is the nature of reality; destruction of ego and embracing of the stable self amount to destruction of ignorance and acquisition of knowledge. The self is like the sun and the ego of ignorance is like darkness, dissipation of which along with acquisition of knowledge of the self reveals the sun and reality. 

Gandhi says (paraphrase) that the self shines in the spiritual heart; the darkness of ignorance envelopes the light that is knowledge. When the darkness of ignorance is destroyed, the light of God is revealed. Knowledge is to be obedient to God and act according to his prompts.

There are four elements in this verse: the knowledge or the wisdom of the self, ignorance, the sun, and the Supreme. Self's inherent quality is light and wisdom, while ignorance is ego and not knowing the duality of self (soul) and body. Once the ignorance is destroyed, the innate wisdom of the self reveals the Supreme, that is Parmataman, as the sun illumines its universe.

5.17:  They, whose intelligence, mind, and faith are steady in Him and who have taken refuge in the supreme, attain that (Supreme abode) from where there is no return, having cleaned their sin by knowledge.

5.18:  A punditah (sage) regards (sees) with an equal eye a humble Brahmin, a cow, an elephant, a dog, and even a dog-eater. 

             The Hindu sacred texts glorify a humble Brahmin because he lives for god, man and other beings. Since he is a preceptor, a man of god, and god’s representative on earth, even his shadow is sacred and therefore, one should not step on his shadow (Garuda Purana 1.96.54).  Brahmins, snakes, Ksatriyas, and Atman (Brahman) should never be insulted (GP 1.96.55). Daily study of Vedas is mandatory for a Brahmin who should never desire for wealth. He can take charity from a king, a pupil or his clan member to satisfy his hunger (GP 1.96.35).

GP (1.213.3) states the duties of a Brahmin are explained in Sruti and Smrti, which are his two eyes. The guiding principles of a Brahmin come from Sruti, Smrti and the virtuous acts of others. Truth, charity, sympathy, renunciation, knowledge, sacrifice, worship and self-control are the eight qualities of the virtuous.  

Krishna Paramatma expresses a similar sentiment in The Uddhava Gita, Dialogue Five, Verse 7. In this instance, Krishna includes things and people in the list. Since things have Self, wanton destruction of things is not respecting the Self.

7 All people and things must become equal

In the eyes of such a devotee ­

Be they spouse, children, home or wealth, Friends or relatives.

The devotee must seek only the Self in all. --Translation by Swami Sarasvati.  

Once Ramakrishna Paramahamsa  (year 1855) was faced with a broken idol of Krishna. The priest carrying the idol of Krishna dropped it and the idol's right foot was broken in several places at the base of the toes. The learned pundits told Ramakrishna Parmahamsa that a broken idol cannot be worshipped and therefore should be immersed (disposed of) in the Ganges River. The proprietress of the temple Rani Rasmani was very sad. RKPH came up with a solution. He told Rani that she would not throw away her son-in-law with a broken leg but take him to a doctor to mend the fracture. RKPH repaired and reattached the toes of the idol and installed it for worship against the established rules of temple worship. Here we see an instance of what Krishna himself said thousands of years ago that "things" and people have Self. He had to break his toes to illustrate his point. Krishna made the priest drop the idol, break his toes and let RKPH repair them and restore the sanctity of the idol, thus showing the equality of people and things, a concept alien to many people.

            All beings are dear not for the sake of being, but for the sake of Self  (Br. UP 2.4.5).  Brahman is all-pervasive both in the insentient and the sentient, and from ameba to man. In man, the sentience expresses the most and in the ameba, it expresses the least. The soul is the same but the embodiment is different and the pundit sees and feels that all living beings are equal. To put it succinctly, there is a metaphysical unity in empirical diversity. This is one of the underlying precepts for ahimsa, noninjury. The souls in the animals have to climb the evolutionary ladder, before these souls can attain to Brahman. The soul has fallen on hard times and the animal soul could have been a human soul in the previous birth; this step down is because of karma; the animal soul has to take on a human body before it can gain moksa. The quality and the nature of Atman in the animals and human beings are the same, but prakrti makes the difference. The pristine nature and intrinsic knowledge of Atman in the animal is no different from that of the human. Mahat or buddhi consists of three entities, namely subconsciousness, consciousness, and superconsciousness. In animals, the subconsciousness or mere consciousness is the instinctive mind, which by its nature lacks reason and intuition. Intuition is defined as present knowledge in continuum from previous existence. By the instinctive mind, a bird builds a nest, and an animal avoids a poisonous plant; fish or animal, when faced with a new set of circumstances alien to its instinct, is dead in the water, or out of its element and resources. The Self or Atman is All-Knower and All-Knowledge; all our knowledge comes from the Atman or Soul. The receptiveness of the animal's brain is low and that of the human brain is high. The animal's inadequate brain can manifest only instinct drawn from the subconscious self, while the human brain, more developed and more receptive, displays not only instincts, but also reason, intellect, and intuition. In yogis more (all-knowing) eternal knowledge percolates through. Another way of putting it is how much a person reflects the all-knowing Atman in him. The more a person reflects, the more of a yogi he becomes. To sum up, animals have instinct and subconsciousness (mere consciousness); man has instinct, reason, intellect, intuition, and self-consciousness. Instinct, reason, intellect, intuition, self-consciousness, and superconsciousness are present in Yogis. Logic (reason), intelligence, and science work well within the scope of the phenomenal world but do not offer any spiritual inspiration or revelation. What is important for an individual self is not being but becoming. Death is not the end of the journey but only the next step for the individual soul to reach its goal of becoming which is superconsciousness.

If I may venture further, I would hazard a guess: The animal's ahamkara (ego-body dominance) is robust, and its buddhi is nonexistent outside the scope of its natural-born instincts. Since buddhi expresses little in an animal, the ahamkara has little oversight from buddhi, the command and control center. Therefore, the animal acts on built-in, hard-wired, instinctive, and programmed reflexes. A fight or flight, and other instinctive behaviors are dominant in animals because of absent development of reason, intuitive mind, and buddhi, as we know in man. That deficiency in animals demands that we are kind to animals. To expand on this, man has less instinct than the animals, but has something that animals do not have. That is reason. Reason is logic. However, reason does not explain all phenomenal events. Reason is within the realm of human self-consciousness; what is beyond is superconsciousness, which carries various labels: Vision, Cosmic Consciousness, Revelation, or Intuition. A yogi has intuition or superconsciousness in quantities that ordinary man does not have. Also more importantly in a Yogi the receptivity of all-knowing eternal knowledge of the Self is high. Reason and a scientific prejudice are an impediment for grasping knowledge of Atman and union with Brahman. Comprehension of Brahman is beyond human reasoning and therefore intuitive intelligence with high receptivity for the all-knowing Self or Atman is a precondition for realization.

The more learned one is, the humbler he becomes. The riper the fruits are, the greater and lower the branches bend: Knowledge is humility. The soul of a dog-eater or a Brahmin is of the same source and therefore can receive SatChitAnanada, Being-Awareness-Bliss. There are no double standards here.

A dog-eater, in this verse, means that he is a person of mixed caste, most likely a Chandala, born of a Brahmin mother and Sudra father.

Mixed castes according to Garuda Purana 1.96-1-73, as narrated by Yajnavalkya.

        ThAkura lists twelve qualities of a Brahmana: truthfulness, control of the senses, austerity, freedom from malice, modesty, tolerance, freedom from envy, sacrifice, charity, fortitude, studying the Vedas, and accepting vows. Page 51- Jaiva Dharma.

5.19:  Even here, they, whose mind, standing firm in equality, conquer the created (phenomenal world). Brahman is flawless and equal in all and so are the persons who stand firm in Brahman. 

Prakrti covers and contaminates the individual soul. Brahman is free from such contamination and thus is the same in everybody. Knowing that Prakrti-free selves are equal, they stand firm in Brahman. 

5.20:  He neither rejoices for getting that which is pleasant nor laments at getting that which is unpleasant. Since he is steady in his intelligence without confusion, the knower of Brahman or God stands firm in God. 

Brahmani Sthitah: Stands firm in Brahman, established in Brahman.

The sweet and sour experiences, compared with those of the birds on the tree, are those of the body and not of the self, and therefore, the stainless self, thus knowing, abides in Brahman. Logic, reason and human intelligence, (intuitive intelligence is a separate entity) do not measure up to understand and know Brahman and the study of scriptures is only a preliminary step in that process. Brahman is the thread that runs through all living entities as in a garland, binds all living entities high and low, and holds and moves the puppets of the world of beings and nature.

Knower of Brahman is one who possesses Brahman knowledge, the knowledge of the Self. Panchadasi (4.55-69) quotes Suresvara, saying that he who pretends to be a knower of Brahman without any moral restraint is like a dog with unclean eating habits. The knower of Truth does not wallow in impurities like a pig in a sty. He who rises above the impurities and desires of the mind is worthy of worship as if he is god. A man who wades in his mind in the river of passions, gets wet with those passions which lead him to attachment, longing, frustration and anger. These can be avoided by meditation on formless Brahman. Once the mind erases the world of matter from its memory, the mind ceases to dwell on the objects and becomes blank like a clean slate; he acts like a dumb and mute person. He is on the doorway opening into the world of Nirvana. If the mind wanders into objects of desire, it is easy for him to bring it back to tranquil state by meditation. He who knows no distraction is a knower of Brahman; he is Brahman Itself. He, who is not concerned whether he is in union with Brahman or not, is Brahman Itself.

5.21:  He, whose soul (mind) has shed all attachments to external contacts (sense objects), enjoys ease in the self. He, with his “self” absorbed in Brahma yoga, enjoys undying (imperishable) happiness or bliss. 

5.22:   Certainly, the accompanying pleasures from contacts with sense objects are seats of sorrow. They have a beginning and an end, O Kaunteya, and the wise do not take any pleasure in them. 

Kaunteya: Son of Kunti, or Arjuna. The fleeting pleasures resulting from the contact of the senses with the sense objects are a source of pain. Knowing that they are Atman, they do not indulge themselves in such pursuits and abide in Brahman. The story of two birds explains this. The first bird is the witness, not eating any fruits. The companion bird eats sweet and sour fruits, alternating between pleasure and pain, until it finds wisdom from its own self and abides in the Self.

5.23:  He, who can endure the flood of desire and anger, before he gives up his body, is a yogin and a happy man. 

Vega: rush, force, flood

Here and now, there is a promise of wisdom and happiness.

Only if one longs for the experience of the self, he gives up desire and anger. There is no implication here that the person controlling himself and experiencing the self will get jivan-mukti, liberation when he is alive. This yogin, having fried his karmic seeds, remains in Kaivalya and Samādhi.

5.24:  He, who finds happiness, pleasure, and the inner light within his self, is a yogin and attains to Brahman and the bliss of Brahman or beatitude of Brahman.

5.25: The sage, who restrains his mind, whose sins come to naught (are destroyed), whose doubts and dualities undergo dissolution (are dissolved), and who shows devotion (is devoted) to the welfare of all beings, attains to Bliss of Brahman.

The sage restrains his mind, he destroys his sins, he clears his doubts and transcends his dualities, he devotes his life to the welfare of all living creatures, and he attains to the bliss of Brahman and Brahman Himself.

5.26: The beatitude of Brahman is imminent in the ascetic, who removed his desire and anger, whose mind exercises restraint, and who has knowledge of the self.

            Matted hair, deerskin, and pious pretension do not make an ascetic. An ascetic in name and epicure in practice, though he professes to know Brahman, is far away from Brahman. Appearance does not make an ascetic, though he wanders naked and shameless; a donkey does the same.  Jackals, rodents and deer live in the forest, eat grass, and drink water: are they ascetics? The frogs and fish take birth, eat and die in Ganga. Are they ascetics? –Garuda Purana, II.49.64-67-68.

This passage has reference to Avadutas, who renounce the world and live on what is offered to them or on leaves, roots and fallen fruits.  

    Avadhûta =  one who has shaken off himself worldly feeling  and obligation.

Here is what Sankara says in Bajagovindam about a false ascetic.

Verse 14.    (There goes) one with matted hair; (here comes) one with shaven head; (there sits) yet another with plucked hair and saffron cloth; the fools sport various disguises. They all claim to "see;"  (What they "see" no one knows and neither he knows what he "sees.") In reality he does not "see."  All this Vesha (disguise) is to fill the belly.

5.27:  Abandoning all sense objects, fixing the gaze between the eyebrows, and keeping the movement of air up and down (Prāna, Apāna) the nostrils in equilibrium within the nostrils, (continued)

Between the eyebrows: antare bhruvoh or Bhru madhya. √ Bhram = Brow. As you see here Brow is derived from Sanskrit word Bhram.

5.28:  The sage, who has controlled his senses, mind, and intellect; who has his highest aim moksa or liberation; and who freed himself from desire, fear, and anger, is forever free (liberated).

Prāna and Apāna breaths: See comment on verse 9 chapter 10 for details.

5.29:  Knowing me thus as the enjoyer of all sacrifices and austerities (Yajana-tapasam) and the supreme Lord of this whole world or universe, the Friend of all living beings, one attains peace. 

Any sacrifice, made to any gods goes to Lord Krishna Himself, since He is the Supreme Lord of all souls, including those of gods.  He is the universe and the enjoyer of all Sacrifices. He is the friend of all the devotees, easy of access, loving as a parent, kind, and compassionate with infinite grace, though inscrutable. He is the knowledge, the Knower, and Doer but the Unknowable too.     

Paramahamsa Babaji says the following on Krishna. Parabrahma Krishnachandra is the one and only Infinite Substance (brhad vastu) and the jivas are his innumerable atomic particles. Like sparks emanating from an undivided fire, the jivas emanate from Krishna, who is the embodiment of innumerable consciousness. The jiva has the full function of consciousness, with which he should seek Krishna.

End of  Chapter Five:  Yoga of Renunciation of  Action