Hello @Azamat, Does the primary and the spare MXs have public IPs directly assigned to them on the WAN interface? If so, you need to use a public IP in the same subnet as the uplink IPs for the virtual IP. If you are using private IPs as WAN IPs for the MXs then you can use a private IP in the same subnet as the WAN IPs as a virtual IP.
Regarding the static route, the static route needs to be created on the upstream core switch pointing towards the Virtual IP it can be public or private depending on what you are using.
Let me know if you have any questions.
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