I dont see a topology attached, but, if your network is layer 2 all the way up to the core, then its better to have the core as the root bridge so that all the other devices could build a shortest path to the core.
not because it has better performance. if you are running RSTP, every bridge is going to generate the BPDU. so, the performance doesnt matter.
if core is the root, every dist will have a ROOT port and ALT port to the core. so, even if 1 port fails, the other one will take over. even if both the links fail, they will converge towards the other dist leading to the core.
if dist-2 is the root, all of its links must be in FWD state.
if you think about upper left triangle, link3-4, link 7 of dist-2 are designated FWD. link3-4 of core is Root FWD. link 7 of Dist-1 is Root FWD. so, which link would be blocked? the link 1-2. similarly, the link5-6 of dist-3 would be blocked. so, all the traffic from all the devices in the network will go through the dist-2. you wont be usign the link between dist-1, dist-3 to core at all.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...