Hi Mach, et. al,
I recall that this question was discussed some time ago and the
clarification came from the original authors of the BFD protocol. The Echo
Reply is optional if there's no error to report. But if the remote LER,
acting as BFD node, does decide to send the Echo Reply it MUST send it
after is sends the first BFD control message.

Regards,
Greg

On Sun, Jul 16, 2017 at 6:58 AM, Mach Chen <mach.c...@huawei.com> wrote:

> Hi BFDers,
>
> We met a multi-vendor interoperate issue recently, it's about whether an
> Echo reply is necessary.
>
> In Section 6 of RFC5884, 2nd paragraph
>
> "... The egress LSR MAY respond with an LSP Ping Echo
>    reply message that carries the local discriminator assigned by it for
>    the BFD session."
>
> >From the above text, my understanding is that an Echo reply is optional,
> the egress LSR can freely to return or not return an Echo reply, and the
> Ingress LSR should not expect there MUST be an Echo reply, but if there is
> one, it should handle it properly.
>
> Is my understanding correct?
>
> Thanks,
> Mach
>
>

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