Rabbi Zeira said that it is derived from here: (Isaiah 4:6) “And there shall be a sukka for shade in the daytime from the heat ..." In a sukka up to twenty cubits high, a person is sitting in the shade of the sukka, i.e., the shade provided from the roofing of the sukkah. In a sukka that is more than twenty cubits high, a person is not sitting in the shade of the sukka (ie, the roofing) but rather in the shade of the walls, (meaning, the height provides constant shade, rendering the shade of the roofing irrelevant). Abaye said to him: But there are those who make their sukka in Ashterot Karnayim, (a place located between two mountains, where the shade of the mountains prevents sunlight from reaching there). Here too, it shouldn't be a sukka, (since he is not sitting in the shade of the roofing but, rather, of the mountains). Rabbi Zeira said to him: There, if one removes the Ashterot Karnayim mountains there would be shade of the sukka. In the other case, if one removes the walls (that were considered too tall), there is no shade by the sukka (since throughout the day sunlight will enter the sukka beneath the roofing from where the walls used to be).
(ב) והיה באחרית הימים נכון יהיה הר ביתיהוה בראש ההרים ונשא מגבעות ונהרו אליו כלהגוים (ג) והלכו עמים רבים ואמרו לכו ונעלה אלהריהוה אלבית אלהי יעקב וירנו מדרכיו ונלכה בארחתיו כי מציון תצא תורה ודבריהוה מירושלם (ד) ושפט בין הגוים והוכיח לעמים רבים וכתתו חרבותם לאתים וחניתותיהם למזמרות לאישא גוי אלגוי חרב ולאילמדו עוד מלחמה (פ)
(2) In the days to come, The Mount of the LORD’s House Shall stand firm above the mountains And tower above the hills; And all the nations Shall gaze on it with joy. (3) And the many peoples shall go and say: “Come, Let us go up to the Mount of the LORD, To the House of the God of Jacob; That He may instruct us in His ways, And that we may walk in His paths.” For instruction shall come forth from Zion, The word of the LORD from Jerusalem. (4) Thus He will judge among the nations And arbitrate for the many peoples, And they shall beat their swords into plowshares And their spears into pruning hooks: Nation shall not take up Sword against nation; They shall never again know war.