The current documentation for cross-cluster live migration's network setup is a bit lacking, especially the part about network configuration.

12.5.2. Configuring a dedicated secondary network for live migration 

The document is currently incomplete, lacking the following essential information:

The L2 network, necessary for cross-cluster live migration, must span all participating clusters. This network needs to be segmented, with each cluster allocated a network block large enough to assign an IP address to every virt-launcher pod running on all its worker nodes.

To illustrate connecting two clusters, A and B, consider the following network configuration:

• Live Migration Network: 192.168.201.0/24
• IP Range for Cluster A: 192.168.201.0/25
• IP Range for Cluster B: 192.168.201.128/25

Each cluster is limited to a maximum of 126 nodes, as each segment is a /25 subnet.

The configuration example for each cluster follows.

Cluster A

apiVersion: k8s.cni.cncf.io/v1
kind: NetworkAttachmentDefinition
metadata:
  name: livemigration-network
  namespace: openshift-cnv
spec:
  config: |
    {
      "cniVersion": "0.3.1",
      "name": "migration-bridge",
      "type": "macvlan",
      "bridge": "br-2001",
      "mode": "bridge",
      "ipam": {
        "type": "whereabouts",
        "range": "192.168.201.0/24",
        "exclude": [
           "192.168.201.128/25"
        ]
      }
    }

Cluster B

apiVersion: k8s.cni.cncf.io/v1
kind: NetworkAttachmentDefinition
metadata:
  name: livemigration-network
  namespace: openshift-cnv
spec:
  config: |
    {
      "cniVersion": "0.3.1",
      "name": "migration-bridge",
      "type": "macvlan",
      "bridge": "br-2001",
      "mode": "bridge",
      "ipam": {
        "type": "whereabouts",
        "range": "192.168.201.0/24",
        "exclude": [
           "192.168.201.0/25"
        ]
      }
    }

The critical difference lies in the exclude configuration, which is necessary to prevent IP address collisions.

Internal discussion: https://redhat-internal.slack.com/archives/C088Z5XL74J/p1764326176822839 
Internal Documentation: Early-access Test Plan: Cross-cluster VM Live Migration