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SNMP load scales with devices times OIDs divided by the interval. The bandwidth is usually trivial; what matters is the data point rate hitting the time-series database and the CPU cost on the polled devices, since SNMP agents are typically single-threaded and a large GetBulk walk can noticeably load a switch's control plane.
SNMP load
OIDs per second = devices x OIDs per device / interval; bandwidth = OIDs per second x bytes per OID x 8
The agent walks its MIB tree in software on the control-plane CPU. Aggressive polling of large tables such as per-VLAN counters can push a switch CPU to saturation.
Streaming telemetry over gRPC or NETCONF, where the device pushes counters at a subscribed interval. It scales far better and delivers finer granularity.