This is done with the 'logging facility'
command on the devices:
After defining your syslog server's IP
address and the level of messaging you want
(I set it to debug because I want to see
on the routers: logging facility local0
on the switches: logging facility local1
Alternative, and more universal, way to do it is to use multiple IPs for
syslog server. Then configure correct syslog server IP on the device.
syslog-ng and others can all do filtering to different destinations
based on the IP where message was received.