RFC 6428, "Proactive Connectivity Verification, Continuity Check, and Remote Defect Indication for the MPLS Transport Profile", November 2011Source of RFC: mpls (rtg)
Errata ID: 6296
Publication Format(s) : TEXT
Reported By: Sudipta Das
Date Reported: 2020-09-29
Section 3.7 says:
In coordinated mode, an implementation SHOULD NOT reset bfd.RemoteDiscr until it is exiting the DOWN state.
It should say:
In coordinated mode, an implementation SHOULD reset bfd.RemoteDiscr upon transitioning to the DOWN state, due to period of a Detection Time passing without the receipt of a valid, authenticated BFD packet from the remote system.
This section seems to imply that when a BFD session, running in coordinated mode, experiences Control Detection Timer expiry, then it SHOULD retain the remote discriminator value *and* SHOULD also transmit the same value in Down packets.
However, in the case when the remote system, configured in Passive role, has its discriminator gets changed (say, after a system reboot which had caused the Detection Timer expiry), it may reject these packets as the received Your Discriminator value is no longer valid for the current session.
So the BFD session would never come back to Up state.
In a second scenario (unrelated to above one), if the local system is configured in Passive role and experiences Control Detection Timer expiry, it may continue transmitting Down packets since the bfd.RemoteDiscr is not reset to zero.