You need at least 2 IPs (one for each WAN) and 3 if you are considering using Virtual IP. Requirements and Best Practices When configuring routed HA, it is critical that both MXs have a reliable connection to each other on the LAN, so the heartbeats of the primary MX can be seen reliably by the spare. To ensure this connection is reliable: The two MXs should be connected to each other through a downstream switch (or ideally, multiple switches) on the LAN to allow for passing VRRP heartbeats. There should be no more than one additional hop between them, and they must be able to communicate on all VLANs. Make sure STP is enabled on the downstream switching infrastructure, as a properly-configured HA topology will introduce a loop on the network. When first configuring routed HA, the spare should be added and configured in the dashboard before the device is physically deployed, so it will immediately fetch its configuration and behave appropriately. Additionally, the following other considerations should be kept in mind: If a virtual IP is being used, each uplink of the two MXs must share the same broadcast domain on the WAN side.
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