Wireless/Redundant Edge Services xl Module Management and Configuration Guide WS.02.xx and greater

14-3
sFlow Agent
Overview
On the Wireless Edge Services xl Module, data sources are RP radios, and “n,”
the packet sampling rate, is configurable per-radio and sampling instance (up
to six per radio). In other words, the module orders radios to send approxi-
mately every “nth” packet to the module’s sFlow agent to be sampled, pack-
aged, and sent to the sFlow receiver or receivers.
Note Only 802.11 data frames are sampled. The sFlow agent does not sample
management and control frames such as beacons.
The sFlow agent packages samples into small datagrams which include,
among other information:
■ sampled packets’ headers
The agent decrypts an encrypted header before sampling it.
■ sampled packets’ inbound or outbound interface
For a wireless network, the inbound interface is the radio.
■ information about the how sampled packets are being forwarded
The extra information in the datagram helps an sFlow receiver to create a
detailed, as well as accurate and up-to-date, picture of network activity. An
advantage of sFlow is that datagrams are nonetheless compact and do not
require a large amount of network bandwidth. The sampled packets are
approximately 7 percent of the original packet size, and several samples can
fit in a single datagram. The same datagrams that include packet samples can
also include counters reported by polled radios (see “Counter Polling by the
sFlow Agent” on page 14-3). Since only some packets are sampled, sFlow
creates very little overhead. For example, if you set the sampling rate to one
packet in 100, flow sampling only adds about .7 percent overhead.
The module uses datagram version 5, and the default datagram size is 1400
bytes. (The size is configurable although you should usually accept the default.
See “Manually Configuring sFlow Receiver Instances Using the Web Browser
Interface” on page 14-8 for more information.)
Counter Polling by the sFlow Agent
In addition to sampling approximately every “nth” packet, the Wireless Edge
Services xl Module’s sFlow agent can sample the counters tracked by radios.
These counters include traffic statistics such as the number of inbound
frames, outbound frames, and retransmitted frames.