Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

If you do not enable sFlow on any port specifically, the global sampling rate is downloaded to that port and is to calculate the port-pipe’s lowest sampling rate. This design supports the possibility that sFlow might be configured on that port in the future. Back-off is triggered based on the port-pipe’s hardware sampling rate.

The default global sampling rate is 32768. The sampling rates are determined as follows:

• If the interface stats are up and the sampling rate is not configured on the port, the default sampling rate is calculated based on the line speed.

• If the interface states are shutdown, the sampling rate is set using the global sampling rate.

• If the global sample rate is non-default, for example 256 bytes, and if the sampling rate is not configured on an interface, the sampling rate of the interface is the global non-default sampling rate, that is 256 bytes.

To avoid the back-off, either increase the global sampling rate or configure all the line card ports with the desired sampling rate even if some ports have no sFlow configured.

Important Points to Remember

• The Dell Networking OS implementation of the sFlow MIB supports sFlow configuration using the snmpset command.
• The Dell Networking OS exports all sFlow packets to the collector. A small sampling rate can equate to many exported packets. A backoff mechanism is automatically applied to reduce this amount. Some sampled packets may be dropped when the exported packet rate is high and the backoff mechanism is about to or is starting to take effect. The dropEvent counter, in the sFlow packet, is always zero.
• Community list and local preference fields are not filled in extended gateway element in the sFlow datagram.
• 802.1P source priority field is not filled in extended switch element in sFlow datagram.
• Only Destination and Destination Peer AS number are packed in the dst-as-path field in extended gateway element.
• If the packet being sampled is redirected using policy-based routing (PBR), the sFlow datagram may contain incorrect extended gateway/router information.
• The source virtual local area network (VLAN) field in the extended switch element is not packed in case of routed packet.
• The destination VLAN field in the extended switch element is not packed in a Multicast packet.
• On the switch, up to 700 packets can be sampled and processed per second.