Three-Tier Network: Core, Aggregation, and Access
The data center is at the foundation of modern software technology, serving a critical role in expanding capabilities for enterprises. The traditional data center uses a three-tier architecture, with servers segmented into pods based on location, as shown in Figure 1-2.
Figure 1-2 Traditional Three-Tier Data Center Design
The architecture consists of core routers, aggregation routers (sometimes called distribution routers), and access switches. Between the aggregation routers and access switches, Spanning Tree Protocol is used to build a loop-free topology for the Layer 2 part of the network. Spanning Tree Protocol provides several benefits. For example, it is simple, and it is a plug-and-play technology requiring little configuration. VLANs are extended within each pod, and servers can move freely within a pod without the need to change IP address and default gateway configurations. However, Spanning Tree Protocol cannot use parallel forwarding paths, and it always blocks redundant paths in a VLAN.
In 2010, Cisco introduced virtual port channel (vPC) technology to overcome the limitations of Spanning Tree Protocol. vPC eliminates spanning tree’s blocked ports, provides active-active uplink from the access switches to the aggregation routers, and makes full use of the available bandwidth, as shown in Figure 1-3. With vPC technology, Spanning Tree Protocol is still used as a failsafe mechanism.
Figure 1-3 Data Center Design Using vPC
vPC technology works well in a relatively small data center environment where most traffic consists of northbound and southbound communication between clients and servers. We will discuss vPCs in detail in Chapter 4, “Port Channels and vPCs.”
Since 2003, with the introduction of virtual technology, the computing, networking, and storage resources that were segregated in pods in Layer 2 in the three-tier data center design can be pooled. This revolutionary technology created a need for a larger Layer 2 domain, from the access layer to the core layer, as shown in Figure 1-4.
Figure 1-4 Data Center Design with Extended Layer 2 Domain
With Layer 2 segments extended across all the pods, the data center administrator can create a central, more flexible resource pool that can be reallocated based on needs. Servers are virtualized into sets of virtual machines that can move freely from server to server without the need to change their operating parameters.
With virtualized servers, applications are increasingly deployed in a distributed fashion, which leads to increased east-west traffic. This traffic needs to be handled efficiently, with low and predictable latency. However, vPC can provide only two active parallel uplinks; therefore, bandwidth becomes a bottleneck in a three-tier data center architecture. Another challenge in a three-tier architecture is that server-to-server latency varies depending on the traffic path used.
A new data center design called the Clos network–based spine-leaf architecture was developed to overcome these limitations. This architecture has been proven to deliver the high-bandwidth, low-latency, nonblocking server-to-server connectivity.
