Port flapping on Cisco IOS devices

Port flapping is a situation in which a physical interface on the switch continually goes up and down, three or more times a second for at least 10 seconds. You might need to identify when port flapping happens on your Cisco devices when doing the following:

• Managing Cisco devices

Prerequisites 

In order to execute this procedure in your environment, the following data, services, or apps are required:

• Cisco Networks Add-on for Splunk Enterprise
• Syslog data collection tier. The Cisco IOS system message logging process uses the syslog protocol to send important messages to remote logging services, such as Splunk. The required add-on expects the data stream to be assigned the syslog sourcetype by the input configuration and will rename it to cisco:ios in the transformation configuration. For details on how the transformation is implemented, see the props.conf and transforms.conf in the add-on and the Splunk documentation. For more information on syslog and Splunk, see the (SYSLOG) Syslog Data Collection section of the Splunk Validated Architectures white paper.

Example

Common causes for port flapping are bad, unsupported, or non-standard cable or other link synchronization issues. The cause for port flapping can be intermittent or permanent. You need a search to identify when it happens on your network so you can investigate and resolve the problem.

NOTE: To optimize the search shown below, you should specify an index and a time range. 

1. Run the following search: 

eventtype=cisco_ios-port_down OR eventtype=cisco_ios-port_up product=IOS
| eval port_state=if(vendor_action="up",1,0)
| stats sparkline(sum(port_state),15m) AS trend count, latest(vendor_action) AS current_port_status BY host,src_interface
| eventstats sum(count) AS host_total BY host
| sort -host_total -count
| fields -host_total
| table host,src_interface,trend,current_port_status,count

Search explanation

The table provides an explanation of what each part of this search achieves. You can adjust this query based on the specifics of your environment.

Result

The following sample result from the search shows the host, source interface, current port status, and the count for the host. The sparkline is not shown below but would be rendered by Splunk.