Users Guide

Implementation Information
Dell Networking sFlow is designed so that the hardware sampling rate is per line card port-pipe and is
decided based on all the ports in that port-pipe.
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.
For example, if port 1 in the port-pipe has sFlow configured with a 16384 sampling rate while port 2 in the
port-pipe has sFlow configured but no sampling rate set, the system applies a global sampling rate of 512 to
port 2. The hardware sampling rate on the port-pipe is then set at 512 because that is the lowest configured
rate on the port-pipe. When a high traffic situation occurs, a back-off is triggered and the hardware sampling
rate is backed-off from 512 to 1024. Note that port 1 maintains its sampling rate of 16384; port 1 is unaffected
because it maintains its configured sampling rate of 16384.:
• If the interface states 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 shut down, the sampling rate is set using the global sampling rate.
• If the global sampling rate is non-default, for example 256, and if the sampling rate is not configured on
the interface, the sampling rate of the interface is the global non-default sampling rate, that is, 256.
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 via snmpset.
• By default, sFlow collection is supported only on data ports. If you want to enable sFlow collection
through management ports, use the management egress-interface-selection and
application sflow-collector commands in Configuration and EIS modes respectively.
• 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.
• Up to 700 packets can be sampled and processed per second.
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