Core issue
The PAGP logging facility reports events that involve the Port Aggregation Protocol (PAgP). PAgP is a protocol used to negotiate EtherChannel links between switches. The switch generates the %PAGP-5-PORTTOSTP or %ETHC-5-PORTTOSTP syslog message whenever a link is detected on a switch port. The switch generates the %PAGP-5-PORTFROMSTP or %ETHC-5-PORTTOSTP syslog message whenever a link is lost on a switch port. These syslogs are normal informational messages that indicate that a port was removed from or added to the spanning tree. Note that channeling does not have to be enabled in order for these messages to be generated. A high number of these messages is an indication that a port loses a link due to a misconfiguration or a connection issue.
In some scenarios, the link flaps when EtherChannel is formed between a Cisco Catalyst switch and a third party switch. Here, the Link-aggregation protocol (LACP) is the only protocol that is supported by both devices.
Note: Do not disable spanning tree when you have have redundant links since this can cause severe network degradation.
Resolution
These messages are normal and of no concern as devices are connected or disconnected from the switch. But, if these messages are seen frequently for a particular port, the link is flapping, which means it loses and regains the link constantly. Only after you determine and fix the root cause can these syslog messages be suppressed by the modification of the logging level for the PAGP facility to four or lower if you issue the set logging level pagp 4 default command. The default logging level for PAgP is five.
These are the most common root causes if these messages occur repeatedly on certain ports:
If the issue is due to misconfigured EtherChannel, refer to the Error-Disabled Because of EtherChannel Misconfiguration section of Recovering From errDisable Port State on the CatOS Platforms and the article How to recover Cisco Catalyst switches that run Cisco IOS system software from the errDisable port state.
If the error occured on the Catalyst 6000/6500 Series Switch, refer to the %PAGP-5-PORTFROMSTP / %PAGP-5-PORTTOSTP section of Common CatOS Error Messages on Catalyst 6500/6000 Series Switches for more information.
For further address port issues, refer to Troubleshooting Switch Port and Interface Problems for more information.
