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Figure 124. sFlow Traffic Monitoring System
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, Dell Networking OS 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 16484.
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.
● sFlow sampling is done on a per-port basis.
● 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.
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sFlow