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Understanding ACI Hardware and Topologies

Chapter Description

ACI is designed to allow small and large enterprises and service providers to build massively scalable data centers using a relatively small number of very flexible topologies. In this sample chapter from CCNP Data Center Application Centric Infrastructure 300-620 DCACI Official Cert Guide, the author team details the topologies with which an ACI fabric can be built or extended.

APIC Clusters

The ultimate size of an APIC cluster should be directly proportionate to the size of the Cisco ACI deployment. From a management perspective, any active APIC controller in a cluster can service any user for any operation. Controllers can be transparently added to or removed from a cluster.


APICs can be purchased either as physical or virtual appliances. Physical APICs are 1 rack unit (RU) Cisco C-Series servers with ACI code installed and come in two different sizes: M for medium and L for large. In the context of APICs, “size” refers to the scale of the fabric and the number of endpoints. Virtual APICs are used in ACI mini deployments, which consist of fabrics with up to two spine switches and four leaf switches.


As hardware improves, Cisco releases new generations of APICs with updated specifications. At the time of this writing, Cisco has released three generations of APICs. The first generation of APICs (M1/L1) shipped as Cisco UCS C220 M3 servers. Second-generation APICs (M2/L2) were Cisco UCS C220 M4 servers. Third-generation APICs (M3/L3) are shipping as UCS C220 M5 servers.

Table 2-2 details specifications for current M3 and L3 APICs.

Table 2-2 M3 and L3 APIC Specifications





2x 1.7 GHz Xeon scalable 3106/85W 8C/11MB cache/DDR4 2133MHz

2x 2.1 GHz Xeon scalable 4110/85W 8C/11MB cache/DDR4 2400MHz


6x 16 GB DDR4-2666-MHz RDIMM/PC4-21300/single rank/x4/1.2v

12x 16 GB DDR4-2666-MHz RDIMM/PC4-21300/single rank/x4/1.2v

Hard drive

2x 1 TB 12G SAS 7.2K RPM SFF HDD

2x 2.4 TB 12G SAS 10K RPM SFF HDD (4K)

Network cards

1x Cisco UCS VIC 1455 Quad Port 10/25G SFP28 CNA PCIE

1x Cisco UCS VIC 1455 Quad Port 10/25G SFP28 CNA PCIE

Note in Table 2-2 that the only differences between M3 and L3 APICs are the sizes of their CPUs, memory, and hard drives. This is because fabric growth necessitates that increased transaction rates be supported, which drives up compute requirements.

Table 2-3 shows the hardware requirements for virtual APICs.

Table 2-3 Virtual APIC Specifications


Virtual APIC


8 vCPUs


32 GB

Hard drive*

300 GB HDD

100 GB SSD

Supported ESXi hypervisor version

6.5 or above

* A VM is deployed with two HDDs.

APIC Cluster Scalability and Sizing

APIC cluster hardware is typically purchased from Cisco in the form of a bundle. An APIC bundle is a collection of one or more physical or virtual APICs, and the bundle that needs to be purchased depends on the desired target state scalability of the ACI fabric.

Table 2-4 shows currently available APIC cluster hardware options and the general scalability each bundle can individually achieve.

Table 2-4 APIC Hardware Bundles

Part Number

Number of APICs

General Scalability

APIC-CLUSTER-XS (ACI mini bundle)

1 M3 APIC, 2 virtual APICs, and 2 Nexus 9332C spine switches

Up to 2 spines and 4 leaf switches


3 M3 APICs

Up to 1200 edge ports


3 L3 APICs

More than 1200 edge ports

APIC-CLUSTER-XS specifically addresses ACI mini fabrics. ACI mini is a fabric deployed using two Nexus 9332C spine switches and up to four leaf switches. ACI mini is suitable for lab deployments, small colocation deployments, and deployments that are not expected to span beyond four leaf switches.

APIC-CLUSTER-M3 is designed for medium-sized deployments where the number of server ports connecting to ACI is not expected to exceed 1200, which roughly translates to 24 leaf switches.

APIC-CLUSTER-L3 is a bundle designed for large-scale deployments where the number of server ports connecting to ACI exceeds or will eventually exceed 1200.

Beyond bundles, Cisco allows customers to purchase individual APICs for the purpose of expanding an APIC cluster to enable further scaling of a fabric. Once a fabric expands beyond 1200 edge ports, ACI Verified Scalability Guides should be referenced to determine the optimal number of APICs for the fabric.

According to Verified Scalability Guides for ACI Release 4.1(1), an APIC cluster of three L3 APICs should suffice in deployments with up to 80 leaf switches. However, the cluster size would need to be expanded to four or more APICs to allow a fabric to scale up to 200 leaf switches.

Each APIC cluster houses a distributed multi-active database in which processes are active on all nodes. Data, however, is distributed or sliced across APICs via a process called database sharding. Sharding is a result of the evolution of what is called horizontal partitioning of databases and involves distributing a database across multiple instances of the schema. Sharding increases both redundancy and performance because a large partitioned table can be split across multiple database servers. It also enables a scale-out model involving adding to the number of servers as opposed to having to constantly scale up servers through hardware upgrades.

ACI shards each attribute within the APIC database to three nodes. A single APIC out of the three is considered active (the leader) for a given attribute at all times. If the APIC that houses the active copy of a particular slice or partition of data fails, the APIC cluster is able to recover via the two backup copies of the data residing on the other APICs. This is why the deployment of a minimum of three APICs is advised. Any APIC cluster deployed with fewer than three APICs is deemed unsuitable for production uses. Note that only the APIC that has been elected leader for a given attribute can modify the attribute.

Figure 2-10 provides a conceptual view of data sharding across a three-APIC cluster. For each data set or attribute depicted, a single APIC is elected leader. Assume that the active copy indicates that the APIC holding the active copy is leader for the given attribute.

Figure 2-10

Figure 2-10 Data Sharding Across Three APICs

For a portion of a database to allow writes (configuration changes), a quorum of APICs housing the pertinent database attributes undergoing a write operation must be healthy and online. Because each attribute in an APIC database is sharded into three copies, a quorum is defined as two copies. If two nodes in a three-node APIC cluster were to fail simultaneously, the remaining APIC would move the entire database into a read-only state, and no configuration changes would be allowed until the quorum was restored.

When an APIC cluster scales to five or seven APICs, the sharding process remains unchanged. In other words, the number of shards of a particular subset of data does not increase past three, but the cluster further distributes the shards. This means that cluster expansion past three APICs does not increase the redundancy of the overall APIC database.

Figure 2-11 illustrates how an outage of Data Center 2, which results in the failure of two APICs, could result in portions of the APIC database moving into a read-only state. In this case, the operational APICs have at least two shards for Data Sets 1 and 3, so administrators can continue to make configuration changes involving these database attributes. However, Data Set 2 is now in read-only mode because two replicas of the attribute in question have been lost.

As Figure 2-11 demonstrates, increasing APIC cluster size to five or seven does not necessarily increase the redundancy of the overall cluster.

A general recommendation in determining APIC cluster sizes is to deploy three APICs in fabrics scaling up to 80 leaf switches. If recoverability is a concern, a standby APIC can be added to the deployment. A total of five or seven APICs should be deployed for scalability purposes in fabrics expanding beyond 80 leaf switches.

If, for any reason, a fabric with more than three APICs is bifurcated, the APIC cluster attempts to recover this split-brain event. Once connectivity across all APICs is restored, automatic reconciliation takes place within the cluster, based on timestamps.

Figure 2-11

Figure 2-11 Impact of APIC Failures in a Five-Node Cluster

What would happen if Data Center 1 in Figure 2-11 failed instead of Data Center 2, and all shards for a specific subset of data resided in Data Center 1 at the time of the outage? In such a scenario, the failure of three APICs could lead to the hypothetical loss of all three shards of a specific subset of data. To ensure that a total loss of a given pod does not result in the loss of all shards for a given attribute, Cisco recommends that no more than two APICs be placed in a single pod.

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