In Genesis 17:20 God promises Abraham (NIV):
And as for Ishmael, I have heard you: I will surely bless him; I will make him fruitful and will greatly increase his numbers. He will be the father of twelve rulers, and I will make him into a great nation.
The promise of the first half of the verse has strong echoes of God's blessing to Abram in Genesis 12:2. The fruitfulness and great numbers echo 17:2 and the promise to make him into a great nation echoes 12:2 and is repeated to Jacob in 46:3. As well, the statement that he will be a father of twelve rulers surely calls to mind Israel.
(Possibly too I see an allusion in Genesis 21 where Ishmael is sent away from Abraham and wanders in the desert wilderness before receiving a bride from Egypt - almost a reverse Exodus?)
Why does Ishmael so closely Israel with respect to the promises traditionally associated with Israel and also with respect to the number twelve? What is the text implying with the comparison?