There is some narration from Prophet Muhammad (PBUH) installed in Masjid an Nabawi in Madinah, justified as "Sahih"(=Correct) or "Hassan"(=Well) in different Sunni references:

النَّبِيِّ صلى الله عليه وسلم قَالَ ‏:‏ ‏ "‏ شَفَاعَتِي لأَهْلِ الْكَبَائِرِ مِنْ أُمَّتِي ‏"‏

The Prophet (PBUH) said: My intercession (shafa'ah) will be for those of my people who have committed major sins. Sunan Abi Dawud 4739, Jami at-Tirmidhi 2435, and also 2436.

What is the idea of Sunni brothers about "Shafa'ah"? Who can do "Shafa'ah"? Is it Shirk to ask prophet's Shafa'ah?