Re: [mpls] Two new drafts on (micro-)BFD over MC-LAG interfaces

Gregory Mirsky <> Sat, 09 April 2016 04:16 UTC

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From: Gregory Mirsky <>
To: Manav Bhatia <>, Greg Mirsky <>
Thread-Topic: [mpls] Two new drafts on (micro-)BFD over MC-LAG interfaces
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Date: Sat, 09 Apr 2016 04:16:23 +0000
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Cc: "Reshad Rahman (rrahman)" <>, "" <>, "" <>, "" <>, "" <>
Subject: Re: [mpls] Two new drafts on (micro-)BFD over MC-LAG interfaces
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Hi Manav,
the use case for the BFD over MC-LAG interfaces that I consider the most important for this drafts to address is to enable sub-second defect detection in order to trigger LACP convergence and, subsequently, switchover within the Redundancy Group in Active-Standby case. Hence both unicast and multicast L3 addresses are quite distant from L2 fast path and processing usually taken by CCM frames. Of course, one can use CFM per LAG Constituent Link, and I have implemented that and it interoperates with another implementation by other vendor, but operators prefer ease of BFD provisioning. Thus I’ve to provide them with ability to monitor MC-LAG and explain that in some cases it may produce false negative when L2 is functional and the problem is in L3, unicast or multicast, engine. As you can see, there’s not much value in continuing argument how much different L3 multicast processing is from L3 unicast as both are different from L2 path. At the end, as I think, it is up to operators to decide whether they are comfortable with this mechanism or would require defect detection at L2.


From: Manav Bhatia []
Sent: Friday, April 08, 2016 5:38 PM
To: Greg Mirsky
Cc: Reshad Rahman (rrahman);;;;;; Gregory Mirsky
Subject: Re: [mpls] Two new drafts on (micro-)BFD over MC-LAG interfaces

Hi Greg,

the update could be in addition of either broadcast or link local multicast or both with appropriate normative language. But I would not agree that these wouldn't work.

Double negatives make it very hard to parse a sentence.

Anyway, why would you NOT agree that this WOULDNT work?

I am telling you that link local multicasts and unicasts are dealt with differently in the data plane, so the data path being up for the former may not necessarily mean that its up for the latter as well. So tell me WHY you think this argument isnt valid? I was the L3 data plane architect in my former company for one of the product lines and i am telling you that in my box, which is very very widely deployed, your scheme will NOT work since i punt all link local packets to the CPU differently. In fact, in some cases even the TX path is different. So sure, u-BFD may very well claim that the link is up, but its possible that there may be no IP connectivity.

Cheers, Manav

Regards, Greg
On Apr 8, 2016 12:34 PM, "Manav Bhatia" <<>> wrote:
Hi Greg,

Not sure i understand how it can "update RFC 7130". Is that by using a link local mcast IP instead of a Unicast IP?

We know that, that wouldnt work.

Cheers, Manav

On Fri, Apr 8, 2016 at 9:44 PM, Gregory Mirsky <<>> wrote:
Hi Reshad,
thank you for your comments. Indeed, RFC 7130 is restricted and thus hardly applicable to MC-LAG case. We realize that if this proposal is adopted it not only enhance applicability on u-BFD but will update RFC 7130.


From: Reshad Rahman (rrahman) [<>]
Sent: Friday, April 08, 2016 8:51 AM
To: Manav Bhatia; Gregory Mirsky
Cc:<>;<>;<>; Alia Atlas (<>);<>;<>

Subject: Re: Two new drafts on (micro-)BFD over MC-LAG interfaces

I agree with Manav, and nothing in RFC7130 seems to preclude using different unicast IP address as destination on different member links.

Reshad (as individual contributor).

From: Rtg-bfd <<>> on behalf of Manav Bhatia <<>>
Date: Friday, April 8, 2016 at 11:04 AM
To: Gregory Mirsky <<>>
Cc: "<>" <<>>, "<>" <<>>, "<>" <<>>, "Alia Atlas (<>)" <<>>, "<>" <<>>, "<>" <<>>
Subject: Re: Two new drafts on (micro-)BFD over MC-LAG interfaces

Hi Greg,

Why cant different micro-BFD packets use the IP address of the MC-LAG end points? Ones going to router 1 will all carry the same unicast IP address. The ones going towards the other router will all carry some other IP address, which would be configured along with the MC-LAG configs.

In fact i would argue that the u-bfd packets going to different routers must use different IP addresses so that you can actually verify the data plane liveliness. Whats the point in sending a contrived IP address if the path that it takes is different from the other regular packets?

Cheers, Manav

On Fri, Apr 8, 2016 at 6:09 PM, Gregory Mirsky <<>> wrote:
Hi Manav,
thank you for sharing insight view of discussions around RFC 7130, extremely helpful.
We believe, and Jeff is co-author of RFC 7130 too, that MC-LAG presents different case and the compromise that you’ve pointed too is justified. We will add more details on the potential differences between unicast and multicast fast paths in the next update.
We are open to the discussion and always welcome comments and alternative proposals.


From: Manav Bhatia [<>]
Sent: Thursday, April 07, 2016 7:39 PM
To: Mach Chen
Cc: Gregory Mirsky;<>;<>;<>;<>;<>; Alia Atlas (<>)
Subject: Re: Two new drafts on (micro-)BFD over MC-LAG interfaces

I believe it had to do with multicast datapath (especially link local) being different from the unicast datapath in most routers. Using link local multicast IP addresses may not necessarily guarantee Unicast IP reachability.

When writing 7130 we spent quite a bit of time ensuring that we dont carve out a special data path for the micro-BFD packets. Using link local would have made it a lot simpler.

And this is where i think the current proposal is flawed -- they use link local multicast to ensure IP unicast reachability which is incorrect.

Cheers, Manav

On Thu, Apr 7, 2016 at 11:16 PM, Mach Chen <<>> wrote:

Hi Greg and all,

I just have quick review on the drafts. If my understanding is correct, the idea is to use multicast destination address other than unicast address when  sending BFD packets over LAG links. And actually this idea has been proposed in (the predecessor of RFC 7130). And at that time, the co-authors of RFC 7130 did discuss the idea of using multicast destination address, but for some reason I forget now(I may need to reiterate the discussions on the archive), the idea was abandoned, although I still think multicast destination address is a smart idea.

Best regards,


From: Rtg-bfd [<>] on behalf of Gregory Mirsky [<>]
Sent: Tuesday, April 05, 2016 6:16
Cc:<>;<>;<>; Alia Atlas (<>)
Subject: Two new drafts on (micro-)BFD over MC-LAG interfaces
Dear All,
two new drafts, related to RFC 7130, were published before the meeting:

·         BFD on MC-LAG interfaces in IP network<>

·         BFD on MC-LAG interfaces in IP/MPLS network<>

Greatly appreciate your reviews, comments, questions and suggestions.


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