With the introduction of Site Recovery Manager 5.0, the customization workflows of the product were improved tenfold as compared to the previous versions of SRM. SRM 5.0 Guest IP customization was a great revolution as this helped reducing the RTO significantly. In the earlier versions of SRM, it use to take a long time for Virtual Machines to get a new IP address during a Test Recovery or Actual Recovery since this was achieved by using utilities like sysprep.
However, with SRM 5.0 the Guest IP customization feature allows you to inject a new IP address to the virtual machine powering on at the DR site within a few seconds. This is done by using the guest OS APIs which are used to push the new IP address as soon as the VM is ready to be powered on. In-fact to achieve this, the virtual machine is first briefly powered on to inject this IP and then it will power on again with the new settings as per the power on priorities and dependencies which you have created in the recovery plan.
The amazing thing is that this can be configured for hundreds of virtual machine by using a pre configured xml file or a convenient GUI option can be used, if you have a smaller environment. To learn about how to configure Guest IP customization refer to the This blog article from VMware Blogs.
Now that you understand the feature, its important that you use and apply this in an environment which can support such a feature. The reason I say this is because of the fact that Guest IP customization does not work on all the Guest Oses which are supported on a vSphere Platform. This will only work on Guest OSes which support Guest IP customization. Though, this feature is supported by most of the Guest Operating Systems, however it is better to check before hand to ensure that you do not face any roadblocks during the implementation. You can get the list of Guest OSes which support Guest Customization on the following link.
In case you get into a situation where you have a Guest OS which is not supported for customization, you would get the following error message in the Recovery Workflow as shown in the screenshot below:-
Error: The Guest operating system "oracleLinux64Guest" is not supported. The value in the "quotes" will be the OS which does not support Customization hence if your recovery workflow has the step to customize the IP of this OS, then the recovery plan will STOP with this error. The end state of the Virtual Machine at the DR site would be in registered mode, however it would be in a powered OFF state.
You can manually power on this VM in the DR site, however the IP address of this machine will remain unchanged from the primary site which could result in a catastrophe, hence please be careful.
Now, let's talk about the remedies and how we can take care of such situations till the time the Guest OS advances and starts supporting the customization option. I believe we can 2 methods to take care of this issue.
Method 1 - We can make this change a manual option, which means that we can easily Add a Message Step to the recovery plan for such Virtual Machines. This message step would be added before the Power On step in the workflow. This message will display that the Virtual machine vNic needs to be disconnected at power on and the virtual machine IP address needs to be changed by the Administrator manually at the time of TEST or Actual RECOVERY. You should also disable the IP Customization step for this machine in the workflow so that the workflow executes successfully.
Method 2 - The second method is more ADMIN friendly as I am going to ask you to script this change and add this to the Post Power on script option in the Recovery Workflow. For this you would need 2 Ethernet configuration files, one for Primary and the other for DR site. Please research and create these files on the basis of the operating system which you have. This would mostly be a Unix flavored OS as most of the Windows Guest OS support Customization. Once you have these files ready, add a script on the Startup of the OS to replace the existing network settings with the new one at the time of , Failover, Failback or Test.
Hope this will help you to tackle such a situation if you come across one.