Re: Service Redundancy using BFD

Ashesh Mishra <> Tue, 28 November 2017 23:17 UTC

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From: Ashesh Mishra <>
To: Greg Mirsky <>
CC: Sami Boutros <>, Ankur Dubey <>, "" <>, Reshad Rahman <>
Subject: Re: Service Redundancy using BFD
Thread-Topic: Service Redundancy using BFD
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Hi Greg,

I’m just trying to understand the use of BFD in this proposal.

I agree with you that 5880 was clear in its scope at the time, but that should not inform the entire scope of BFD in the future.


From: Greg Mirsky <>;
Date: Tuesday, November 28, 2017 at 5:06 PM
To: Ashesh Mishra <>;
Cc: Sami Boutros <>;, Ankur Dubey <>;, ""; <>;, Reshad Rahman <>;
Subject: Re: Service Redundancy using BFD

Hi Ashesh,
I believe that the abstract of RFC 5880 is very clear of what is the goal of BFD:

   This document describes a protocol intended to detect faults in the

   bidirectional path between two forwarding engines, including

   interfaces, data link(s), and to the extent possible the forwarding

   engines themselves, with potentially very low latency.  It operates

   independently of media, data protocols, and routing protocols.

Applications, e.g. routing protocols, residing on the BFD node may use notifications of BFD state changes to trigger their own processes. An implementation may use BFD state changes to draw conclusions of state of its remote peer but, I strongly believe, BFD is not intended to verify anything but path continuity between two nodes and, to some extent, proper functioning of the forwarding engines at BFD nodes.


On Tue, Nov 28, 2017 at 1:14 PM, Ashesh Mishra <<>> wrote:
Thanks for the response, Sami. I think our disconnect lies in the definition of a service. From a BFD perspective, I expect the service to be established across two nodes, at the very least, so that BFD can monitor its liveness. Can you elaborate on

-          What, in the context of this draft, a service is?

-          How does BFD signal for a service that it is not monitoring the liveness for?


From: Sami Boutros <<>>
Date: Tuesday, November 28, 2017 at 1:23 PM
To: Ashesh Mishra <<>>, Ankur Dubey <<>>, "<>" <<>>
Cc: Reshad Rahman <<>>

Subject: Re: Service Redundancy using BFD

Hi Ashesh,

Thanks for your comments.

For your first comment the draft applies to both single hop or what you call interface BFD and multi hop BFD too. And yes the per service could be per interface too if this is a single hop BFD, we can clarify that in the draft.

For your second comment, I am not sure I understand. The service will be active only on one node, if the service is associated with the whole node, then the BFD session is monitoring the node liveness. And when the service is associated with an interface the BFD session will monitor the interface connectivity as well. So, a primary service can’t be active at the 2 node endpoints hosting the BFD session.


From: Ashesh Mishra <<>>
Date: Tuesday, November 28, 2017 at 4:04 AM
To: Ankur Dubey <<>>, "<>" <<>>
Cc: Reshad Rahman <<>>, Sami Boutros <<>>
Subject: Re: Service Redundancy using BFD

Hi Ankur,

This is a good proposal to pursue within the BFD-wg.

Couple of comments:

-          BFD can only signal this diag code for the interface that it is monitoring (the IP next hop, MPLS LSP, etc.). You mention per-service (which I assume means per-service-per-interface) failover in the draft but it may be worthwhile defining behavior on per-service-type-per-interface as well.

-          There still needs to be a method for the primary and backup pairs (two BFD end-points on primary service and two on backup service) to communicate with each other (primary-to-primary and backup-to-backup) if the service is active or standby. This is useful in the scenario when the primary cannot communicate with backup nodes (it is a failure condition after all).

Again, at 10k ft, I like the idea of signaling active/standby using BFD.


From: Rtg-bfd <<>> on behalf of Ankur Dubey <<>>
Date: Monday, November 27, 2017 at 9:47 PM
To: "<>" <<>>
Cc: Reshad Rahman <<>>, Sami Boutros <<>>
Subject: Service Redundancy using BFD

Hi all,

Please review and provide comments for the following draft:<>

Summary of draft:

This draft proposes a new BFD diag code via which a node running a BFD session with another node, can inform the other node after a BFD session times out, that it didn’t go down and did live through the failure.

Such notification is useful for a set of nodes providing Active/Standby redundancy. When these nodes are running multiple L2/L3/L4-L7 services  in non-revertive mode of redundancy, the standby node taking over as active for non-revertive services after BFD times out needs to indicate in the BFD packet that it outlived the other failed old active node. The new diag code will be used for this purpose. When this diag code is set in the BFD packets, it will provide an indication to the failed old active node that it MUST NOT activate the non-revertive services when it comes up.

For providing a per service level failover, a node activating certain non-revertive services needs to indicate that it is Active ONLY for those non-revertive services. This can be done by using a unique bitmap where each bit position is uniquely identifying a service. This unique bitmap is configured on all nodes by a network controller. When there is at least one non-revertive service for which a node is not active AND it is active for at least 1 non-revertive service, this node will set bits identifying the active services in the bitmap and send it in the payload of the BFD packet.