Re: [Lsr] Open issues with Dynamic Flooding

[Tony Przygienda <[e-mail]>]

On Tue, Mar 5, 2019 at 11:12 AM Robert Raszuk <[e-mail]> wrote:

>
> > Slow convergence is obviously not a good thing
>
> Could you please kindly elaborate why ?
>
> With tons of ECMP in DCs or with number of mechanism for very fast data
> plane repairs in WAN (well beyond FRR) IMHO any protocol *fast convergence*
> is no longer a necessity. Yet many folks still talk about it like the only
> possible rescue ...
>
>
>
Agree on FRR in WAN but it seems quite a complexity to drag that into DC
fabrics (again, depeneding what topologies talk about) especially due to
the effect I outline below.

ECMP in DCs when used AFAIS is often caused by walking the 10G-50G curve
where everybody has different economic sweet-spots AFAIS. Sometimes 100G.
It's often LAG'ed (due to amount of control plane state AFAIU). LAG'ing
generally creates interesting problems itself and I expect whole thing to
settle somewhere with 20-50G to the server over either very short copper or
multimode. I do not think it's a particularly cool architecture to suggest
"LAG every link so it's 1:1 protected".

Wide fanout is common & will get wider AFAIS. This is cool for failures in
one direction but doesn't prevent you from blackholing on the "far end"
until you get control plane reconvergence (or do FRR on a spine but
honestly, how do you do that, bow-tieing over top of fabric or doing
harmonica routing over multi-homed servers?)

Then, slow convergence means that new stuff shows up slowly which is
probably not very desirable and in case of failures that really need
control plane rerouting causes longer duration blackholes as well. Both
seem undesirable .. Not mentioning mobility even ...

my

[image: CodeCogsEqn(17).gif]   cents ;-)

--- tony