In HA (active-passive) mode, SNMP monitoring tools show passive status as down even if Link State is configured as Auto.
When interfaces are configured as Layer-2, L2/L3 Dot-1Q interfaces or Virtual Wire (VWire), the interfaces of the passive device show as down in order for the traffic to flow through only the active device.
When Layer-3 interfaces are used, the passive device's interfaces remain up if the Passive Link State is configured as Auto.
Solution:
This is expected behavior:
HA passive device with link state Auto has its interfaces in a special 'up' state that allows link state negotiation with the neighbouring device, but they are not connected to the system as such. For the routing engine and SNMP monitoring, the interfaces are in a down state as they are not allowed to participate in network activity in any way until the system becomes active.