HOW DO YOU ENABLE LOGGING ON A CISCO 2811?

Logging should be on by default, but if not you will want to console or telnet or ssh into the router and get into enable mode and then go into configuration mode with “config term” and then type the command “logging on” followed by the command “logging buffered 4096 debugging” and I’d also suggest the command “service timestamps log datetime msec”.
These commands will 1) enable logging which it already should have been by default and typically will not show up in the output of a show run. 2) set up a 4KB log buffer to copy log messages to an internal buffer, first in first out so only the most recent 4KB of messages will be in there. Increase the size accordingly if needed. 3) configures the timestamps in the log messages to reflect the date and time (as opposed to using the “uptime” keyword) and gives it millisecond granularity.
The above assumes your 2811 has the correct date and time, if not, use the clock set command, such as “clock set 21:15:00 25 April 2009″ for example. Unless maybe you’ve pointed it to an NTP server using the “ntp server ” command.
Once these are configured, exit config mode, save the config, and use the show logging command to watch for your circuit bouncing. If you’re in a telnet or ssh session and not connected directly to the console port, also give the “terminal monitor” command so see the log output as if you were on the console port.

for console connection
router(config)# line cons 0
router(config-line)# logg on
for telnet connection
router#terminal monitorhttp://www.computerfreetips.com/Cisco_ro…

depending on the IOS of the router
logging enabled
logging buffered 10000
then type
show log
then to view the output in ‘real time’ from SSH / Telnet
term mon

Have you tried “term mon”