CCLM compared with OD-CCLM · NCP-MCI-7.5 study notes

Two features live migrate running VMs between clusters with no downtime, and they have different prerequisites. Cross-cluster live migration (CCLM) is a planned failover of synchronously protected VMs. On-demand cross-cluster live migration (OD-CCLM) needs no protection at all.

Attribute	CCLM	OD-CCLM
Protection needed	Synchronous replication schedule	None; no DR setup needed
Started from	Recovery plan, Failover, Planned Failover, Live Migrate Entities, at the recovery AZ	Compute, VMs, Migrate Across Clusters, at the primary AZ
Hypervisor	AHV	AHV; ESXi is not supported
Latency	RTT under 5 ms for synchronous replication	RTT 40 ms or less recommended
Network	L2 stretch across AZs; plan with Stretch Networks	L2 extension in most cases; none within the same VPC subnet
Flow Network Security VMs	Fail; export policies and import at the recovery AZ	Retained within one Prism Central
Managed (IPAM) VLAN	Fails	Not supported
Credential Guard	Not supported	Supported
Software	Synchronous requirements	AOS 6.7, pc.2023.3; for vTPM, AOS 6.8, pc.2024.1

Shared requirements: the same AHV version on both clusters, and a destination that supports all source CPU features unless advanced processor compatibility is enabled. Both fail for VMs with attached volume groups or with memory overcommit.

OD-CCLM also fails for VMs in a consistency group, with replication factor 1, with GPU passthrough, with vNUMA, or when paused. It fails while a Prism Central, AHV or LCM upgrade runs, and an AHV upgrade fails during a migration. Other rules:

The VM must be powered on, and hotplug changes during the copy are fatal.
A VM with NGT needs an empty IDE or SATA CD-ROM.
The destination subnet must use the same prefix as the source.
The workflow needs IPv4.
Across Prism Central instances only the local admin can migrate, and becomes the VM’s owner. Within one instance, non-administrators need Allow_Cross_Cluster_VM_Migration and view permissions.
Categories are not preserved if they do not exist at the destination AZ.
Before AOS 7.3, open 2030, 2036, 2073 and 2090 by hand.

After CCLM, pause or disable synchronous replication before shutting down the original primary, or the VMs freeze trying to re-stretch. Check eligibility with this command:

nutanix@cvm$ acli vm.get <VM_NAME> | grep 'allow_live_migrate'

Common trap: Assuming OD-CCLM traffic is protected like DR replication - on-wire encryption is on by default for replication, but data over the wire during OD-CCLM is not encrypted.

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