Re-Defining Core and Access: A New, Two-Tier Network Model

In the past, when we designed, built, and operated networks as a collection of devices (routers, switches, and firewalls) we defined our network architecture in te …

In the past, when we designed, built, and operated networks as a collection of devices (routers, switches, and firewalls) we defined our network architecture in terms of physical layers. The three-tiered Core, Aggregation/Distribution, and Access model is familiar to every network engineer. Server virtualization and new application frameworks have forced us to reconsider this model. Instead of a multi-tier hierarchical design, we have found folded-Clos (spine-leaf) networks much more efficient at moving large quantities of packets from anywhere to anywhere. In order to keep up with the speed of virtualized compute and storage, we’ve adopted virtualized networks that run as an overlay (with the physical Clos network becoming an underlay). Visualizing the network in this way gives us a new 2-tier model. Instead of trying to conceptualize the physical network into an outdated hierarchy, we can now look at the entire logical network platform as a two tier system. The (spine-leaf) underlay is the Core layer switch and the overlay is the Access layer router. This is super helpful when we want to decide where network functions should live. The Core is still there to move packets, fast, and the Access is there to handle routing and policy as well as to provide additional features and functions.