data center design

Virtual Data Center Design


data center design

Virtual Data Center Design

In today’s rapidly evolving digital landscape, data centers support the growing demand for storage, computing power, and connectivity. However, traditional data centers face challenges like limited scalability, high operating costs, and physical space constraints. Many organizations are turning to virtual data centers as a more efficient and flexible solution to address these issues. In this blog post, we will delve into the concept of virtual data centers, exploring their benefits, key components, and significance in shaping the future of data management.

A virtual data center (VDC) is a software-defined infrastructure that leverages virtualization technologies to pool and abstract computing resources, storage, and networking capabilities. Unlike traditional data centers, which rely on physical servers and hardware, VDCs enable organizations to create, manage, and provision virtual machines (VMs) and applications on demand.


  • Gaining Efficiency

Deploying multiple tenants on a shared infrastructure is far more efficient than having single tenants per physical device. With a virtualized infrastructure, each tenant requires isolation from all other tenants sharing the same physical infrastructure.

For a data center network design, each network container requires path isolation, for example, 802.1Q on a shared Ethernet link between two switches, and device virtualization at the different network layers, for example, Cisco Application Control Engine ( ACE ) or Cisco Firewall Services Module ( FWSM ) virtual context. To implement independent paths with this type of data center design, you can create Virtual Routing Forwarding ( VRF ) per tenant and map the VRF to Layer 2 segments.

  • Example: Virtual Data Center Design. Cisco.

More recently, the Cisco ACI network enabled segmentation based on logical security zones known as endpoint groups. Where security constructs known as contracts are needed to communicate between endpoint groups. The Cisco ACI still uses VRFs, but they are used differently. Then we have the Ansible Architecture that can be used with Ansible variables to automate the deployment of the network and security constructs for the virtual data center. This brings consistency and will eliminate human error.


Before you proceed, you may find the following posts helpful for pre-information:

  1. Context Firewall
  2. Virtual Device Context
  3. Dynamic Workload Scaling
  4. ASA Failover
  5. Data Center Design Guide


Data Center Network Design

Key Virtual Data Center Design Discussion Points:

  • Introduction to Virtual Data Center Design and what is involved.

  • Highlighting the details of VRF-lite and how it works.

  • Critical points on the use of virtual contexts and how to implement them.

  • A final note on load disributon and appliciation tier separation. 


Back to basics with data center types.

Numerous kinds of data centers and service models are available. Their category relies on several critical criteria. Such as whether one or many organizations own them, how they serve in the topology of other data centers, and what technologies they use for computing and storage. There are main types of data centers include:

  • Enterprise data centers.
  • Managed services data centers.
  • Colocation data centers.
  • Cloud data centers.

You may build and maintain your own hybrid cloud data centers, lease space within colocation facilities, also known as colos, consume shared compute and storage services, or even use public cloud-based services.


Benefits of Virtual Data Centers:

1. Scalability: Virtual data centers offer unparalleled scalability, allowing businesses to quickly expand or contract their infrastructure based on their evolving needs. With the ability to provision additional resources in real time, organizations can quickly adapt to changing workloads, ensuring optimal performance and reducing downtime.

2. Cost Efficiency: Virtual data centers significantly reduce operating costs by eliminating the need for physical servers and reducing power consumption. Consolidating multiple VMs onto a single physical server optimizes resource utilization, improving cost efficiency and lowering hardware requirements.

3. Flexibility: Virtual data centers allow organizations to deploy and manage applications across multiple cloud platforms or on-premises infrastructure. This hybrid cloud approach enables seamless workload migration, disaster recovery, and improved business continuity.

Critical Components of Virtual Data Centers:

1. Hypervisor: At the core of a virtual data center lies the hypervisor, a software layer that partitions physical servers into multiple VMs, each running its operating system and applications. Hypervisors enable the efficient utilization of hardware resources and facilitate VM management.

2. Software-Defined Networking (SDN): SDN allows organizations to define and manage their network infrastructure through software, decoupling network control from physical devices. This technology enhances flexibility, simplifies network management, and enables greater security and agility within virtual data centers.

3. Virtual Storage: Virtual storage technologies, such as software-defined storage (SDS), enable the pooling and abstraction of storage resources. This approach allows for centralized management, improved data protection, and simplified storage provisioning in virtual data centers.


Data center network design: VRF-lite

VRF information from a static or dynamic routing protocol is carried across hop-by-hop in a Layer 3 domain. Multiple VLANs in the Layer 2 domain are mapped to the corresponding VRF. VRF-lite is known as a hop-by-hop virtualization technique. The VRF instance logically separates tenants on the same physical device from a control plane perspective.

From a data plane perspective, the VLAN tags provide path isolation on each point-to-point Ethernet link that connects to the Layer 3 network. VRFs provide per-tenant routing and forwarding tables and ensure no server-server traffic is permitted unless explicitly allowed.

virtual and forwarding


Service Modules in Active/Active Mode

Multiple virtual contexts

The service layer must also be virtualized for tenant separation. The network services layer can be designed with a dedicated Data Center Services Node ( DSN ) or external physical appliances connected to the core/aggregation. The Cisco DSN data center design cases use virtual device contexts (VDC), virtual PortChannel (vPC), virtual switching system (VSS), VRF, and Cisco FWSM and Cisco ACE virtualization. 

This post will look at a DSN as a self-contained Catalyst 6500 series with ACE and firewall service modules. Virtualization at the services layer can be accomplished by creating separate contexts representing separate virtual devices. Multiple contexts are similar to having multiple standalone devices.

The Cisco Firewall Services Module ( FWSM ) provides a stateful inspection firewall service within a Catalyst 6500. In addition, it offers separation through a virtual security context that can be transparently implemented as Layer 2 or as a router “hop” at Layer 3. The Cisco Application Control Engine ( ACE ) module also provides a range of load-balancing capabilities within a Catalyst 6500.


FWSM  features

 ACE features

Route health injection (RHI)

Route health injection (RHI)

Virtualization (context and resource allocation)

Virtualization (context and resource allocation)

Application inspection

Probes and server farm (service health checks and load-balancing predictor)

Redundancy (active-active context failover)

Stickiness (source IP and cookie insert)

Security and inspection

Load balancing (protocols, stickiness, FTP inspection, and SSL termination)

Network Address Translation (NAT) and Port Address Translation (PAT )


URL filtering

Redundancy (active-active context failover)

Layer 2 and 3 firewalling

Protocol inspection


You can offer high availability and efficient load distribution with a context design. The first FWSM and ACE are primary for the first context and standby for the second context. The second FWSM and ACE are primary for the second context and standby for the first context. Traffic is not automatically load-balanced equally across the contexts. Additional configuration steps are needed to configure different subnets in specific contexts.


Virtual Firewall and Load Balancing
Diagram: Virtual Firewall and Load Balancing


Compute separation

Traditional security architecture placed the security device in a central position, either in “transparent” or “routed” mode. All inter-host traffic had to be routed and filtered by the firewall device located at the aggregation layer before communication could occur. This works well in low virtualized environments when you have few VMs. Still, a high-density model ( heavily virtualized environment ) forces us to reconsider firewall scale requirements at the aggregation layer.

It is recommended to deploy virtual firewalls at the access layer to address the challenge of VM density and the ability to move VMs while keeping their security policies. This creates intra and inter-tenant zones and enables finer security granularity within single or multiple VLANs.


Application tier separation

The Network-Centric model relies on VLAN separation for three-tier application deployment for each tier. Each tier should have its VLAN in one VRF instance. If VLAN-to-VLAN communication needs to occur, traffic must be routed via a default gateway where security policies can enforce traffic inspection or redirection.

vShield ( vApp ) virtual appliance can inspect inter-VM traffic among ESX hosts, and layers 2,3,4 and 7 filters are supported. A drawback of this approach is that the FW can become a choke point. Compared to the Network-Centric model, the Server-Centric model uses separate VM vNICs to daisy chain tiers together.


Data center network design with Security Groups

The concept of Security groups replacing subnet-level firewalls with per-VM firewalls/ACLs. With this approach, there is no traffic tromboning or single choke points. It can be implemented with Cloudstack, OpenStack ( Neutron plugin extension ), and VMware vShield Edge. Security groups are elementary; you assign VMs and specify filters between groups. 

Security groups are suitable for policy-based filtering but don’t consider other functionality where data plane states are required for replay attacks. Security groups give you echo-based functionality and should be good enough for current TCP stacks that have been hardened over the last 30 years. But if you require full stateful inspection and do not regularly patch your servers, then you should implement a complete stateful-based firewall.


data center design

Matt Conran
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