Hello David!
The Engine ID is a feature specific to SNMPv3, and was introduced as part of its enhanced security model. The engine ID uniquely identifies each SNMPv3 entity, be it an SNMP agent or an SNMP manager. The uniqueness of the ID is crucial for security purposes, keeping in mind that SNMPv3 has added security features compared to its predecessors. The Engine ID is used in the generation of security parameters, and it helps to ensure that the communication is indeed with the correct device. Without unique Engine IDs, there could be potential for security breaches or miscommunication. Some more info about this feature and how its configuration affects the SNMP setup can be found in this Cisco documentation:
When a device boots up, SNMP assigns a particular index to identify each individual interface on that network device. These indexes are used by SNMP to identify the specific interface for which it’s collecting statistics.
Shuffling of these indexes can occur during a reboot or a minor software upgrade. This is especially true if virtual interfaces, SVIs, tunnels, or loopback interfaces have been configured over the lifetime of the device. When a reboot occurs, the order that the indexes have been assigned may change.
This means that the index previously assigned to a particular interface might be assigned to a different interface after the reboot or upgrade. This can cause confusion and inaccuracies in your network monitoring, as the SNMP manager might collect data for the wrong interface.
The command snmp-server ifindex persist (or snmp ifmib ifindex persist in the running config) is used to prevent this shuffling. It ensures that the same index is assigned back to the same interface even after a reboot or software upgrade, hence maintaining accuracy in your network monitoring.
I hope this has been helpful!
Laz