Defining a Sampling Interval

Use the ip flow-sampling-mode packet-interval command to define the packet-sampling interval for the virtual router. The sampling interval specifies the rate at which the virtual router samples J-Flow information. This rate is used for all interfaces that have J-Flow enabled. After you enable J-Flow on an interface, the virtual router samples one packet at the specified packet interval. You can specify an interval in the range 1–4,000,000,000 packets.

When you use the ip flow-sampling-mode packet-interval command to define the packet-sampling interval for Gigabit Ethernet interfaces configured on the ES2 10G LM (line module) with either the ES2-S1 GE-8 IOA or the ES2-S2 10GE PR IOA on E120 routers and E320 routers, the J-Flow application makes the following internal adjustments to achieve better performance on the ES2 10G LM, regardless of the packet-sampling interval that you configure:

• J-Flow adjusts the maximum sampling interval to 8,388,608, which is the decimal equivalent of 0x800000.
• J-Flow changes the packet-sampling value to the closest integer that is a power of two and that is less than or equal to the configured value.

For performance reasons, J-Flow applies these adjustments to the sampling interval only for the interfaces configured on the ES2 10G LM on the virtual router. The configured sampling interval does not change for interfaces not configured on the ES2 10G LM on the virtual router.

When the data rate increases on a given interface, J-Flow packet sampling might not be able to maintain the configured sampling rate and might drop the intended sampled packets. If this occurs, you can address the issue by reducing the sampling rate.

Note: For all modules except the ES2 10G LM on the E120 router and the E320 router, packet sampling occurs individually for each processor. Because the router distributes packets over multiple processors, sampling occurs when each processor reaches the specified packet interval. Even though each flow is sampled, the flow sample is not necessarily cached because of system constraints.

ip flow-sampling-mode packet-interval

• Use to define the J-Flow packet-sampling interval.
• Specify a packet-sampling interval in the range 1–4000000000 packets; the default value is 4000000000.
• Specifying an interval less than 10 sets a very high sampling rate that can severely degrade performance. The lower the packet-sampling interval you configure, the faster the sampling rate.
• For information about the effects of using the ip flow-sampling-mode packet-interval command for the ES2 10G LM with either the ES2-S1 GE-8 IOA or the ES2-S2 10GE PR IOA on E120 routers and E320 routers, see Defining a Sampling Interval .
• Example—Samples 1 out of 50 packets from the line module on which the interface resides

  host1(config)#ip flow-sampling-mode packet-interval 50

• Use the no version to return the sampling interval to its default value, 4 billion.
• See ip flow-sampling-mode packet-interval.

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