Re: [Idr] I-D Action: draft-ietf-idr-rs-bfd-03.txt

[John Scudder <[e-mail]>]

On Jul 3, 2017, at 5:31 PM, Job Snijders <[e-mail]> wrote:
> 
> On Mon, 3 Jul 2017 at 23:18, John G. Scudder <[e-mail] <mailto:[e-mail]>> wrote:
> 
>> On Jul 3, 2017, at 5:10 PM, Job Snijders <[e-mail] <mailto:[e-mail]>> wrote:
>> On Mon, 3 Jul 2017 at 22:53, Randy Bush <[e-mail] <mailto:[e-mail]>> wrote:
>> > I don't follow. If the route server facilitates you in setting up BFD to
>> > all "nexthops of interest", the route server's job is done, right? Any RS
>> > peer has the ability to locally verify reacahbility across the fabric and
>> > route accordingly.
>> 
>> route servers are for the small folk who can not do full mesh, let alone
>> add path and other fancy things.  the rs IS their control plane, period.
>> 
>> If a device can't do add-path, can we really expect these devices to do fancy things like opportunistic "route server assisted" BFD to all next-hops of interest?
>> 
>> If we operate under the assumption that these clients can't support receiving multiple paths through add-path, I envision a somewhat ugly cadence:
>> 
>> 1) Hey RS, can't reach A
>> 2) Ok, here try reaching A via B
>> 3) hey RS, I can't reach B either
>> Etc, etc
> 
> If that cadence happens, they haven't implemented what's in rs-bfd, right? So why would even #1 happen?
> 
> 
> Perhaps my understanding of the draft is flawed,  but rs-bfd (currently) specifies the concept of relaying information from RS participant back to the RS about who the participant can reach or not.

Yes, but I think the part you are overlooking is that the RS asks the client to track connectivity to all candidate next-hops, whether or not the RS is currently sending the client a route with that next hop. 

> So #1 can only happen if rs-bfd is implemented,

Yes, but.

> and the cadence would be a logical consequence if add-path is not supported. 

That's the error. Numbers 2 and 3 wouldn't happen if rs-bfd were implemented as written, because in step zero the RS would have said "hey client, please use BFD towards A and B" and in step 1, the client would say "hey RS, can't reach A, also can't reach B".

--John