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.

https://networklessons.com/cisco/ccie-routing-switching-written/is-is-route-leaking