Hello,
I would suggest verifying the below -
1) Configure an out-of-band contract allowing SNMP traffic. SNMP traffic typically uses UDP port 161 for SNMP requests.
2) Configure the APIC out-of-band IP addresses in the 'mgmt' tenant. Although the out-of-band addresses are configured during APIC setup, the addresses must be explicitly configured in the 'mgmt' tenant before the out-of-band contract will take effect.
Configuration guide for reference - https://www.cisco.com/c/en/us/td/docs/dcn/aci/apic/6x/basic-configuration/cisco-apic-basic-configuration-guide-60x/m_management.html?bookSearch=true#Cisco_Concept.dita_76d8e54d-5ffa-46f1-9b62-6d6f1e7f4e39
Cisco ACI MIB Support List - https://www.cisco.com/c/dam/en/us/td/docs/Website/datacenter/aci/mib/mib-support.html
Since SNMP walk went through, I would recommend opening a Cisco technical support case to investigate further.
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