Final answer:
The command used to filter log messages to a SYSLOG server is 'logging trap'. It allows administrators to choose the severity level of log messages that are sent to the SYSLOG server from Cisco IOS devices.
Step-by-step explanation:
The command that allows you to filter log messages to a SYSLOG server in networking devices such as routers and switches is 'logging trap'. This command is used on devices running Cisco's IOS to set the logging level of messages that are sent to a SYSLOG server. The levels are ordered from 0 to 7, with 0 being emergencies and 7 being debugging messages. Using the 'logging trap' command, administrators can specify the type or level of messages they want to be sent to their SYSLOG server, helping to manage the volume and relevance of logged data.