ACI Clos architecture

[dan.almasan]

What is the reason for not connecting host/external routers to the spine? Is this requierement Cisco specific? In case fabrics from other vendor allow connecting the DC gateway to the spines, is there a reason not to do it?

[stcorry]

In ACI, this is a requirement. I am not aware of all the reasons why this is a hard requirement, but overall this is so that the Spine layer is only doing L3 Routing and little else. This is partly due to predictability and limiting the number of possible combinations of designs. The Spine layer also acts as an aggregation for high speed links which the leaf switches will mostly have and that you don't necessarily want to give to servers. 

In other fabric architectures, typically only switches will connect to the spines, whether it's another layer of leafs for aggregation or another layer of spines (super spines!). 

[dpita]

Hello. 

This may be a newer feature so i cannot comment much on it, but take a look at this white paper describing some new options for connecting back to a DC core via a N7k or ASR9k

http://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/white-paper-c11-736899.html

You will see "Connectivity Options" showing the DC gateway/core connected to the spines for a particular setup for DCI

But to Steve's point. up until now the spines do not connect to anything else but the Leafs. Definitely not connected to endpoints!