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Cisco Application Centric Infrastructure Overview

Today I am going to talk about the Active/Active data centers implementation while using Cisco ACI infrastructure. As you guys know about it that Active/Active datacenter are executed in numerous architectures. 

The most usual design contains splitting or delivering the application or service in two different data centers. This method uses a Global Site Load Balancer (GSLB) to straight the client to the truthful data center that comprises the application host based on DNS load balancing policy.

There are few challenges with this approach:
  • The DNS time to live (TTL) value need time out before the user will be transmitted to the new location (data center) for the application.
  • Layer 2 extension is compulsory between the data centers and a solution for Source Network Address Translation (SNAT).
  • Layer 2 extension has its own confronts including traffic issues and asymmetrical traffic patterns.

The Active / Active ACI Stretched fabric architecture addresses these challenges by using policy that spans between the data centers.

Let's talk about the approach which we are doing to achieve overlay networks in previous years. In the last previous years, there have been a numeral of overlay protocols implemented to address the suboptimal traffic routing in an active/active data center environment. 

A network overlay usually delivers either a Layer 2 or Layer 3 service. Some of the usual data center Layer 2 network overlays are Fabric Path (TRILL), OTV, and VXLAN. Layer 3 overlays comprise of GRE, BGP MPLS VPNs, and LISP. The overlay provides a straightforward service of encapsulating a frame or packet and spreading over the underlay network to the remote overlay tunnel endpoint. 

When it reaches the remote overlay tunnel endpoint, it is un encapsulated and redirected. The overall objective is to provide a service (layer 2/3) that would not be native to the Ethernet/IP underlying network all while hiding the underlay to the two endpoints communicating over the overlay network. 

OTV is a commonly deployed overlay to connect two data centers at layer 2 which allows for in service workload mobility. LISP is a layer 3 overlay that fixes some of the challenges of inbound routing correction described previously.

Cisco Application Centric Infrastructure Overview
The Cisco Application Centric Infrastructure (ACI) fabric consists of three components: a controller, policy and network infrastructure. The central controller of the Application Policy Infrastructure Controller (APIC), implements network policy for forwarding packets on switches in a spine and leaf architecture. The APIC summaries the network infrastructure and provides a central policy engine.

Configuration of the fabric and implementation of policy is through the northbound REST API interface of the APIC. Multiple controllers are attached to separate leaf switches for availability. Configuration changes made on one controller are communicated and stored across all controllers in the fabric.

Fig 1.1- Cisco ACI approach

Switches assist either a spine or leaf role. Leaf switches can also have additional sub roles within the ACI fabric; border or transit leaf. A border leaf switch has a Layer 3 connection to external networks.

Recent releases of ACI software support disjointed leaf switches, leaf switches that do not have connections to every spine within the fabric. A disjointed leaf can be a transit leaf, connecting two spines located in unlike physical locations. 

By connecting the two spines together with the transit leafs, the two locations are controlled with a single policy by a cluster of APICs distributed across both locations. In addition to supporting transit leaf switches, the 40 Gigabit Long range QSFP optics provide connectivity of up to 30 kilometers.