Re: Alvaro Retana's Discuss on draft-ietf-isis-sbfd-discriminator-02: (with DISCUSS)

Acee,

On Wed, Nov 18, 2015 at 06:49:18PM +0000, Acee Lindem (acee) wrote:
> > We'll give the authors a chance to reply, but basically:
> > - discriminators must already be domain-wide unique.
> > - The uniqueness property tends to imply provisioning.
> > - Thus the foreknowledge for the mapping is probably implied.
> 
> I hate to be a killjoy, but if everything is pre-provisioned, what is the benefit of advertising the S-BFD discriminators in the IGPs? 

I did say "imply". :-)

There's also a slight difference between provisioning for purposes of making
sure things are unique domain wide and putting a copy of that mapping on
every single node.  That's at least one reason to distribute in the IGP.

And the other case would be something like derived from router-id.  

> > See section 5, second to last paragraph for reinforcement that it's a local
> > matter.
> > 
> > The better questions are "why would you give a given node more than one
> > discriminator?"  The typical case is likely to be for scaling the number of 
> > BFD sessions on that node.
> 
> For this use case, pre-provisioning could be avoided with a standard algorithm to choose among multiple discriminators. 

Even for this case, I'd hesitate to be too proscriptive.  Trying to describe
an algorithm to provide load-balancing domain-wide with unknown loads is
likely to be wrong... and hard to compete with "pick one at random" in many
cases.

-- Jeff