C-8 is the only HE hub to use an SoC based on the Zigbee 3.0 protocol; while this sounds like an upgrade (there's literally no performance benefit) for the typical HE user with legacy Zigbee HA profile devices, there's a significant downside: devices that disconnect might be unable to transparently rejoin the mesh as they normally would if joined to a non Zigbee 3.0 hub.
It's not disconnects that are the issue; they happen even in a properly functioning mesh for a variety of reasons: routine battery replacement, temporarly signal blockage, momentary power interruption to their parent router... Zigbee devices are designed to recognize when their link is lost and go through a rejoin process to reconnect. We expect this to happen without user intervention; that's the experience most of us have with C-3 through C-7 hubs (at least once the mesh is stable).
The improvement Zigbee 3.0 provides-- at least in a mesh consisting of all 3.0 devices (along with multiple profile support, irrelevant to legacy Zigbee users)-- is fixing security vulnerabilites caused by use of a default link key (the link key is used during joins/rejoins to encrypt transfer of a unique network key used for subsequent communication). Legacy devices all use the same 'well known' link key during initial join and insecure rejoins to obtain the unique netkwork key. 'Pure' Zigbee 3.0 networks fix that problem but create new issues when legacy and 3.0 devices must interoperate.
The Zigbee 3.0 security model doesn't use a default link key for device joins-- instead a unique link key can be embedded in the 3.0 device (printed on the device itself and entered 'out of band' when the device is initially joined) . If the C-8 adhered to this security model, it would then be incompatible with older HA devices which can only join use the default link key.
Unfortunately meshes that must use legacy devices with a Zigbee 3.0 coordinator face issues-- documented by Si Labs in one of their app notes Silicon Labs Community :
My guess is that the C-8 takes this approach:
"Using the default link key as a transient link key (a link key with a timeout after which it will no longer work). This allows HA devices to join a network for a small window, minimizing security vulnerabilities, but it can cause problem if a legacy device associated using the default link key detaches from the network, it won't be able to rejoin using an unsecured rejoin"
Per Silicon Labs, to get the same level of reliability (that is, rejoins without user intervention) that we've been used to, but still using the C-8 with legacy devices, would require:
" Using the default link key as the primary link key within the network. This solves any HA interoperability issues, but with the concerns of wide open security".
Personally I'd be fine with that as a configuration option; fortunately the only one motivated to hack my Zigbee network is me. I have a C-8 but it's been unused; frankly it would be my last choice (after my also yet to be deployed C-7; flaky Z-Wave kept that one on the shelf) to replace my nearly 100% rock solid C-3 as Zigbee coordinator.
