A router interface transitioning to the down state will generate a log entry with a severity level of 5 if syslog has been enabled on a router. Syslog is a management protocol that can be used to transmit logging information from a device to a syslog server. When syslog is enabled on a router, logging messages are generated by the router and sent to the console or to a syslog server that is used to view and process the syslog messages. Log files that are generated by routers are categorized into one of the following severity levels:
Level 0 - Emergency
Level 1 - Alert
Level 2 - Critical
Level 3 - Errors
Level 4 - Warnings
Level 5 - Notifications
Level 6 - Informational
Level 7 – Debugging
A router interface transitioning to the down state would cause a log entry to be generated with a severity level of 5. A severity level of 5 indicates that a normal but significant event has occurred. System restart messages are also displayed at this level.
A log entry with a severity level of 6 indicates an informational message. An informational message is generated when an event such as a packet being denied as a result of matching an access control list (ACL) entry occurs. Reload request messages are also displayed at this level.
The Cisco IOS software failing to load would generate a log entry with a severity level of 0. A severity level of 0 indicates that the system is unusable; an emergency condition has occurred that has prevented the router from functioning.
If a router interface receives an invalid packet type, a log entry will be created with a severity level of 7. A severity level of 7 indicates a debugging message.