Why the change of heart re "optimized" forwarding to enable ARP flooding?

[RedNectar]

Hi experts,

For years I have explaned to Cisco cutomers how Cisco "optimizes" ACI broadcast flooding by default by treating ARPs more like IP packets rather than broadcasts.

So I was suprised to notice that in ACI v4.2 the default setting when choosing "Optimized" forwarding is to enabling "ARP flooding" rathter than well... what I thought was "optimising" ARP flooding. Reading the release notes I see that this is only the case for the GUI implementation, and not the CLI or API (which is in fact how I noticed the change because the BD I'd created usign the API did not match the one created using the GUI)

Now I do understand that a moving a silent bare metal host from one location to another can cause the "old" version of optimisation some grief, but feel there must be some other compelling reason for changing the default setting.

Can anyone enlighten me on the full reason for the change of heart, and the new way of explaining what "optimized" forwarfing means?

[Claudia de Luna]

@RedNectar Thanks for posting this for awareness, if nothing else!  I'm hoping you get a reply as I'd like to understand the rationale for that as well.  I always try to make a point of making sure any BD that can be optimized is optimized after a migration but the answer to this may change that approach.

[richmond]

Hi Chris,

I'm keen to hear from Cisco on this one as well. In the mean time I can offer some speculation.

In brownfield deployments we often don't know if there are devices present that rely on ARP flooding updates for their MAC to IP bindings. It's safer to set the BD to ARP flood to ensure that all ARP gets flooded to every endpoint in the BD rather than having ARP optimised. Maybe there were TAC cases where people had migrated and had issues post-migration so Cisco have changed the default to be the safer option for migrations?

I also noticed another change in 4.2 where VMM bindings in the GUI are defaulting to On-Demand/Pre-Provision rather than On-Demand/Immediate.

[RedNectar]

Thanks to both @richmond & @Claudia de Luna for your contibutions. I'm still hoping to get a better reply from someone at Cisco.

I am aware of and can accept @richmond's point that " It's safer to set the BD to ARP flood to ensure that all ARP gets flooded to every endpoint in the BD rather than having ARP optimised" but that statement actually EMPHAISISES my point rather than explains it.

You see @richmond you used the term optimised in the sense that I USED to understand it, not Cisco's new definition.

I would have no trouble if Cisco had changed the default settings had changed to having ARP flooding enabled and the default Forwarding behaviour changed to custom.  But that is NOT what has happened. 

What has happened is that the DEFINITION of "optimized" has changed. Now when I choose "optimized" forwarding, nothing gets "optimised".  It just doesn't make sense.

At the end of the day, the Forwarding setting in the Create Bridge Domain dialogue is only a GUI enhancement, and does not directly relate to any single setting in ACI - it only appears when creating the Bridge Domain.

So I've made a call to the ACI developers to remove the Forwarding option. It simply does NOT make sense any more.