MME-S1 signaling failure

[snabeel03]

Different sites on different times shows low accessibility.
› Accessibility establishment goes into three phases: 1. RRC
establishment phase 2. S1 establishment phase 3. ERAB
establishment phase.
› Accessibility KPI is low due to S1 phase failure.
› Sites are not failing everyday and the failure happen on
different times.
Main failure messages S1_INTERFACE_DOWN
•Followed by EVEN_VALUE_S1_RESET

Any insights on the causes and resolution for this?
Vendor is E*******.