OSPF - Down (DN) Bit

The DN (Down) bit is a field found within the Options section of a Link-State Advertisement (LSA) in the payload of an OSPF message. This bit is present in all LSAs as well as in Database Description (DB) messages. The primary purpose of the DN bit is to prevent routing loops in OSPF networks, particularly in scenarios involving Virtual Routing and Forwarding (VRF) Lite.

This DN bit is used by the capability vrf-lite command.

Links:

https://forum.networklessons.com/t/vrf-lite-configuration-on-cisco-ios/1166/109?u=lagapidis

https://forum.networklessons.com/t/when-and-where-to-use-capability-vrf-lite/14877/2