Designing your network for disaster recovery

This article is directed to IT professionals who are responsible for architecting, implementing, and supporting business continuity and disaster recovery (BCDR) infrastructure, and who want to leverage Microsoft Azure Site Recovery (ASR) to support and enhance their BCDR services. This article discusses practical considerations in structuring network at disaster recovery site, be it Azure or another on-premises site.


Azure Site Recovery (ASR) is a Microsoft Azure service that orchestrates the protection and recovery of your virtualized applications for business continuity disaster recovery (BCDR) purposes. This document is intended to guide the reader through the process of designing the networks, focusing on architecting IP ranges and subnets on the disaster recovery site, when replicating virtual machines (VMs) using Site Recovery.

Furthermore, this article demonstrates how Site Recovery enables architecting and implementing a multisite virtual datacenter to support BCDR services at time of test or disaster.

In a world where everyone expects 24/7 connectivity, it is more important than ever to keep your infrastructure and applications up and running. The purpose of Business Continuity and Disaster Recovery (BCDR) is to restore failed components so the organization can quickly resume normal operations. Developing disaster recovery strategies to deal with unlikely, devastating events is very challenging. This is due to the inherent difficulty of predicting the future, particularly as it relates to improbable events, and the high cost to provide adequate measures of protection against far-reaching catastrophes.

Crucial for BCDR planning, Recovery Time Objective (RTO) and Recovery Point Objective (RPO) must be defined as part of a disaster recovery plan. When a disaster strikes the customer’s data center, using Azure Site Recovery, customers can quickly (low RTO) bring online their replicated virtual machines located in either the secondary data center or Microsoft Azure with minimum data loss (low RPO).

Failover is made possible by ASR which initially copies designated virtual machines from the primary data center to the secondary data center or to Azure (depending on the scenario), and then periodically refreshes the replicas. During infrastructure planning, network design should be considered as potential bottleneck that can prevent you from meeting company RTO and RPO objectives.

When administrators are planning to deploy a disaster recovery solution, one of the key questions in their minds is how the virtual machine would be reachable after the failover is completed. ASR allows the administrator to choose the network to which a virtual machine would be connected to after failover. If the primary site is Azure or it is an on-premises site managed by a VMM server then this is achieved by using Network Mapping. Learn more about Network Mapping in Azure to Azure DR and Network Mapping from VMM

While designing the network for the recovery site, the administrator has two choices:

  • Use a different IP address range for the network at recovery site. In this scenario the virtual machine after failover will get a new IP address and the administrator would have to do a DNS update.
  • Use same IP address range for the network at the recovery site. In certain scenarios administrators prefer to retain the IP addresses that they have on the primary site even after the failover. In a normal scenario an administrator would have to update the routes to indicate the new location of the IP addresses. But in the scenario where a stretched subnet is deployed between the primary and the recovery sites, retaining the IP addresses for the virtual machines becomes an attractive option. Stretching a subnet from an on-premises network to an Azure network or between two Azure networks is not possible.

When administrators are planning to deploy a disaster recovery solution, one of the key questions in their mind is how the applications will be reachable after the failover is completed. Modern applications are almost always dependent on networking to some degree, so physically moving a service from one site to another represents a networking challenge. There are two main ways that this problem is dealt with in disaster recovery solutions. The first approach is to maintain fixed IP addresses. Despite the services moving and the hosting servers being in different physical locations, applications take the IP address configuration with them to the new location. The second approach involves completely changing the IP address during the transition into the recovered site. Each approach has several implementation variations which are summarized below.

While designing the network for the recovery site, the administrator has two choices:

Option 1: Retain IP addresses

From a disaster recovery process perspective, using fixed IP addresses appears to be the easiest method to implement, but it has a number of potential challenges which in practice make it the least popular approach. Azure Site Recovery provides the capability to retain the IP addresses in all scenarios. Before one decides to retain IP, appropriate thought should be given to the constraints it imposes on the failover capabilities. Let us look at the factors that can help you to make a decision to retain IP addresses, or not. This can be achieved in two ways, by using a stretched subnet or by doing a full subnet failover.

Stretched subnet

Here the subnet is made available simultaneously in both primary and DR locations. In simple terms this means you can move a server and its IP (Layer 3) configuration to the second site and the network will route the traffic to the new location automatically. This is trivial to deal with from a server perspective but it has a number of challenges:

  • From a Layer 2 (data link layer) perspective, it will require networking equipment that can manage a stretched VLAN, but this has become less of a problem as it is now widely available. The second and more difficult problem is that by stretching the VLAN the potential fault domain is extended to both sites, essentially becoming a single point of failure. While this is an unlikely occurrence, it has happened that a broadcast storm started but could not be isolated. We have found mixed opinions about this last issue and have seen many successful implementations as well as “we will never implement this technology here”.
  • Stretched subnet is not possible if you are using Microsoft Azure as the DR site.

Subnet failover

It is possible to implement subnet failover to obtain the benefits of the stretched subnet solution described above without stretching the subnet across multiple sites. Here any given subnet would be present at Site 1 or Site 2, but never at both sites simultaneously. In order to maintain the IP address space in the event of a failover, it is possible to programmatically arrange for the router infrastructure to move the subnets from one site to another. In a failover scenario the subnets would move with the associated protected VMs. The main drawback to this approach is in the event of a failure you have to move the whole subnet, which may be OK but it may affect the failover granularity considerations.

Let’s examine how a fictional enterprise named Contoso is able to replicate its VMs to a recovery location while failing over the entire subnet. We will first look at how Contoso is able to manage their subnets while replicating VMs between two on-premises locations, and then we will discuss how subnet failover works when Azure is used as the disaster recovery site.

Failover from on-premises to Azure

Azure Site Recovery (ASR) allows Azure to be used as a disaster recovery site for your virtual machines.

Let’s examine a scenario where a fictional company named Woodgrove Bank has on-premises infrastructure hosting their line of business applications, and they are hosting their mobile applications on Azure. Connectivity between Woodgrove Bank VMs in Azure and on-premises servers is provided by a site-to-site (S2S) Virtual Private Network (VPN) or ExpressRoute. S2S VPN allows Woodgrove Bank’s virtual network in Azure to be seen as an extension of Woodgrove Bank’s on-premises network. This communication is enabled by S2S VPN between Woodgrove Bank edge and Azure virtual network. Now Woodgrove wants to use ASR to replicate its workloads running primary Azure region to another Azure region. This option meets the needs of Woodgrove, which wants an economical DR option and is able to store data in public cloud environments. Woodgrove has to deal with applications and configurations which depend on hard-coded IP addresses, hence they have a requirement to retain IP addresses for their applications after failing over to another region in Azure.

Woodgrove has decided to assign IP addresses from IP address range (, to its resources running in Azure.

For Woodgrove to be able to replicate its virtual machines to Azure while retaining the IP addresses, an Azure Virtual Network needs to be created. It should be an extension of the on-premises network so that applications can failover from the on-premises site to Azure seamlessly. Azure allows you to add site-to-site as well as point-to-site VPN connectivity to the virtual networks created in Azure. When setting up your site-to-site connection, Azure network allows you to route traffic to the on-premises location (Azure calls it local-network) only if the IP address range is different from the on-premises IP address range, because Azure doesn’t support stretching subnets. This means that if you have a subnet on-premises, you can’t add a local-network in the Azure network. This is expected because Azure doesn’t know that there are no active VMs in the subnet and that the subnet is being created only for DR purposes. To be able to correctly route network traffic out of an Azure network the subnets in the network and the local-network must not conflict.

Before Subnet Failover

Before failover

To help Woodgrove fulfill their business requirements, we need to implement the following workflows:

  • Create an additional network, let us call it Recovery Network, where the failed-over virtual machines would be created.
  • To ensure that the IP for a VM is retained after a failover, go to the Configure tab under VM properties, specify the same IP that the VM has on-premises, and then click Save. When the VM is failed over, Azure Site Recovery will assign the provided IP to the virtual machine.

Network properties

Once the failover is triggered and the virtual machines are created in the Recovery Network with the desired IP, connectivity to this network can be established using a Vnet to Vnet Connection. If required this action can be scripted. As we discussed in the previous section about subnet failover, even in the case of failover to Azure, routes would have to be appropriately modified to reflect that has now moved to Azure.

After Subnet Failover

After failover

If you don't have a 'Azure Network' as shown in the picture above. You can create a site to site vpn connection between your 'Primary Site' and 'Recovery Network' after the failover.

Failover to a secondary on-premises site

Let us look at a scenario where we want retain the IP of each of the VMs and fail-over the complete subnet together. The primary site has applications running in subnet When the failover happens, all the virtual machines that are part of this subnet will be failed over to the recovery site and retain their IP addresses. Routes will have to be appropriately modified to reflect the fact that all the virtual machines belonging to subnet have now moved to the recovery site.

In the following illustration the routes between primary site and recovery site, third site and primary site, and third site and recovery site will have to be appropriately modified.

The following pictures shows the subnets before the failover. Subnet is active on the Primary Site before the failover and becomes active of the Recovery Site after the failover

Before Failover

Before failover

The picture below shows networks and subnets after failover.

After Failover

After failover

In your secondary site is on-premises and you are using a VMM server to manage it then when enabling protection for a specific virtual machine, ASR will allocate networking resources according to the following workflow:

  • ASR allocates an IP address for each network interface on the virtual machine from the static IP address pool defined on the relevant network for each System Center VMM instance.
  • If the administrator defines the same IP address pool for the network on the recovery site as that of the IP address pool of the network on the primary site, while allocating the IP address to the replica virtual machine ASR would allocate the same IP address as that of the primary virtual machine. The IP is reserved in VMM but not set as failover IP on the Hyper-v host. Failover IP on a Hyper-v host is set just before the failover.

If the same IP is not available, ASR would allocate some other available IP address from the defined IP address pool.

After the VM is enabled for protection you can use following sample script to verify the IP that has been allocated to the virtual machine. The same IP would be set as Failover IP and assigned to the VM at the time of failover:

    $vm = Get-SCVirtualMachine -Name <VM_NAME>
    $na = $vm[0].VirtualNetworkAdapters>
    $ip = Get-SCIPAddress -GrantToObjectID $na[0].id

In the scenario where virtual machines use DHCP, the management of IP addresses is completely outside the control of ASR. An administrator has to ensure that the DHCP server serving the IP addresses on the recovery site can serve from the same range as that of the primary site.

Option 2: Changing IP addresses

This approach seems to be the most prevalent based on what we have seen. It takes the form of changing the IP address of every VM that is involved in the failover. A drawback of this approach requires the incoming network to ‘learn’ that the application that was at IPx is now at IPy. Even if IPx and IPy are logical names, DNS entries typically have to be changed or flushed throughout the network, and cached entries in network tables have to be updated or flushed, therefore a downtime could be seen depending upon how the DNS infrastructure has been setup. These issues can be mitigated by using low TTL values in the case of intranet applications and using Azure Traffic Manger with ASR for internet based applications

Changing the IP addresses - Illustration

Let us look at the scenario where you are planning to use different IPs across the primary and the recovery sites. In the following example we also have a third site from where the applications hosted on primary or recovery site can be accessed.

Different IP - Before Failover

In the picture above there are some applications hosted in subnet subnet on the primary site, and they have been configured to come up on the recovery site in subnet after a failover. VPN connections/network routes have been configured appropriately so that all three sites can access each other.

As the picture below shows, after failing over one or more applications, they will be restored in the recovery subnet. In this case we are not constrained to failover the entire subnet at the same time. No changes are required to reconfigure VPN or network routes. A failover and some DNS updates will make sure that applications remain accessible. If the DNS is configured to allow dynamic updates then the virtual machines would register themselves using the new IP once they start after a failover.

Different IP - After Failover

After failing-over the replica virtual machine might have an IP address that isn’t the same as the IP address of the primary virtual machine. Virtual machines will update the DNS server that they are using after they start. DNS entries typically have to be changed or flushed throughout the network, and cached entries in network tables have to be updated or flushed, so it is not uncommon to be faced with downtime while these state changes take place. This issue can be mitigated by:

  • Using low TTL values for intranet applications.
  • Using Azure Traffic Manger with ASR for internet based applications.
  • Using the following script within your recovery plan to update the DNS Server to ensure a timely update (The script is not required if the Dynamic DNS registration is configured)

      $Record = Get-DnsServerResourceRecord -ZoneName $zone -Name $name
      $newrecord = $record.clone()
      $newrecord.RecordData[0].IPv4Address  =  $IP
      Set-DnsServerResourceRecord -zonename $zone -OldInputObject $record -NewInputObject $Newrecord

Changing the IP addresses – DR to Azure

The Networking Infrastructure Setup for Microsoft Azure as a Disaster Recovery Site blog post explains how to setup the required Azure networking infrastructure when retaining IP addresses isn’t a requirement. It starts with describing the application and then look at how to setup networking on-premises and on Azure and then concluding with how to do a test failover and a planned failover.

Next steps

Learn how Site Recovery maps source and target networks when a VMM server is being used to manage the primary site.