IS-IS - up-down bit
The up/down bit is a bit set within the IS-IS LSP that is used in IS-IS Route Leaking.
It is used to prevent the re-redistribution of networks back into the level 2 database. When a prefix is redistributed like this, the router that does the redistribution will set the distribution up/down bit.
Links:
https://networklessons.com/cisco/ccie-routing-switching-written/is-is-route-leaking