Thanks for the responses.
Here's the whole tree and the bad limb with the rotted part indicated. My dilemma is that I'm inclined to leave the limb as a food source for the birds unless the problem will continue to spread. Also I wonder if I need to do additional treatment beyond removing the limb.
Any ID or advice? Will it spread? Should I remove the limb? This is in North Texas. I think on a Red Oak. The limb is alive, it had leaves and acorns this year.
Thanks a bunch.