IS-IS - attached bit

The attached bit, or the ATT bit is a bit set within the IS-IS LSP that is used to indicate to a Level 1-2 router to generate a default route.

Once a level 1-2 router is connected to another area, it will set a special bit in its level 1 LSP called the attached bit. When a level 1 router sees this, it will generate a default route that is pointed to the level 1-2 router.

https://networklessons.com/cisco/ccie-routing-switching-written/introduction-to-is-is