CISCO Elephant Flow Detection User Guide
- June 13, 2024
- Cisco
Table of Contents
CISCO Elephant Flow Detection User Guide
Elephant flows are extremely large (in total bytes), continuous flows set up by a TCP (or other protocols) flow measured over a network link. By default, elephant flows are those larger than 1 GB/10 seconds. They can cause performance duress in Snort cores. Elephant flows are not numerous, but they can occupy a disproportionate share of the total bandwidth over a period of time. They can lead to problems, such as high CPU utilization, packet drops, and so on.
From management center 7.2.0 onwards (Snort 3 devices only), you can use the elephant flow feature to detect and remediate elephant flows, which helps to reduce system stress and resolve the mentioned issues.
- About Elephant Flow Detection and Remediation, on page 1
- Elephant Flow Upgrade from Intelligent Application Bypass, on page 1
- Configure Elephant Flow, on page 2
About Elephant Flow Detection and Remediation
You can use the elephant flow detection feature to detect and remediate elephant flows. The following remediation actions can be applied:
- Bypass elephant flow–You can configure elephant flow to bypass Snort inspection. If this is configured, Snort does not receive any packet from that flow.
- Throttle elephant flow–You can apply rate-limit to the flow and continue to inspect flows. The flow rate is calculated dynamically and 10% of the flow rate is reduced. Snort sends the verdict (QoS flow with 10% less flow rate) to the firewall engine. If you choose to bypass all applications including unidentified applications, you cannot configure the throttle action (rate-limit) for any flow.
Note For the elephant flow detection to work, Snort 3 must be the detection engine.
Elephant Flow Upgrade from Intelligent Application Bypass
Intelligent Application Bypass (IAB) is deprecated from version 7.2.0 onwards
for Snort 3 devices.
For devices running 7.2.0 or later, you must configure elephant flow settings
under the Elephant Flow Settings section in the AC policy (Advanced settings
tab).
Post-upgrade to 7.2.0 (or later), if you are using a Snort 3 device, the elephant flow configuration settings will be picked and deployed from the Elephant Flow Settings section and not from the Intelligent Application Bypass Settings section, so if you have not migrated to Elephant Flow configuration settings, your device will lose the elephant flow configuration upon the next deployment.
The following table shows the IAB or elephant flow configurations that can be applied to version 7.2.0 or later and to version 7.1.0 or earlier that are running Snort 3 or Snort 2 engines.
Configure Elephant Flow
You can configure elephant flow to take actions on elephant flows, which helps resolve issues, such as system duress, high CPU utilization, packet drops, and so on.
**Attention :** Elephant flow detection is not applicable for prefiltered, trusted, or fast-forwarded flows, which do not process through Snort. As elephant flows are detected by Snort, elephant flow detection is not applicable for encrypted traffic.
Procedure
Step 1
Figure 1: Configure Elephant Flow Detection
Step 2 The Elephant Flow Detection toggle button is enabled by default. You
can configure the values for flow bytes and flow duration. When they exceed
your configured values, elephant flow events are generated.
Step 3 To remediate elephant flows, enable the Elephant Flow Remediation
toggle button.
Step 4 To set the criteria for remediation of the elephant flow, configure the
values for CPU utilization %, duration of fixed time windows, and packet drop
%.
Step 5 You can perform the following actions for elephant flow remediation
when it meets the configured criteria:
a. Bypass the flow—Enable this button to bypass Snort inspection for selected
applications or filters. Choose from:
• All applications including unidentified applications—Select this option to
bypass all the application traffic. If you configure this option, you cannot
configure the throttle action (rate-limit) for any flow.
• Select Applications/Filters—Select this option to select the applications or
filters whose traffic you want to bypass; see Configuring Application
Conditions and Filters.
b. Throttle the flow—Enable this button to apply rate-limit to the flow and
continue to inspect flows. Note that you can select the applications or
filters to bypass Snort inspection and throttle the remaining flows.
Note
Automatic removal of throttle from a throttled elephant flow occurs when the
system is out of duress, that is, the percentage of Snort packet drops is
lesser than your configured threshold. Consequently, rate limiting is also
removed.
You can also manually remove throttling from a throttled elephant flow, using
the following threat defense commands:
• clear efd-throttle <5-tuple/all> bypass—This command removes throttling from
the throttled elephant flow and bypasses Snort inspection.
• clear efd-throttle <5-tuple/all>—This command removes throttling from the
throttled elephant flow and Snort inspection continues. Elephant flow
remediation is skipped after using this command.
For more information about these commands, see the Cisco Secure Firewall
Threat Defense Command Reference.
Attention
Taking action on elephant flows (bypass and throttle the flow) is not supported on Cisco Firepower 2100 series devices.
Step 6 In the Remediation Exemption Rule section, click Add Rule to configure
L4 access control list (ACL) rules for flows that must be exempted from
remediation.
Step 7 In the Add Rule window, use the Networks tab to add the network
details, that is the source network and the destination network. Use the Ports
tab to add the source port and the destination port.
If an elephant flow is detected and it matches the rules that are defined, an
event is generated with the reason as Elephant Flow Exempted in the Reason
column header of Connection Events.
Step 8 In the Remediation Exemption Rule section, you can view the flows that
are exempt from the remediation action.
Step 9 Click OK to save the elephant flow settings.
Step 10 Click Save to save the policy.
What to do next
Deploy configuration changes; see Deploy Configuration Changes.
After configuring your elephant flow settings, monitor your connection events
to see if any flows are detected, bypassed, or throttled. You can view this in
the Reason field of your connection event. The three reasons for elephant flow
connections are:
• Elephant Flow
• Elephant Flow Throttled
• Elephant Flow Trusted
Attention Enabling elephant flow detection alone does not cause generation of connection events for elephant flows. If a connection event is already logged for another reason and the flow is also an elephant flow, then the Reason field contains this information. However, to ensure that you are logging all elephant flows, you must enable connection logging in the applicable access control rules.
Refer to Cisco Secure Firewall Elephant Flow Detection for more information.
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