Administrator 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.
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.
Enabling and Disabling sFlow
By default, sFlow is disabled globally on the system.
Use the following command to enable sFlow globally.
sFlow 831