>What level of software is required for NTP support in
>a cisco router?

9.21 or above.

>Which cisco routers support NTP?

It is a software feature exclusively. Anything that supports
9.21 or 10 will run NTP (when running that s/w).

>How do I set it up?

The basic hook is:
        ntp server <host> [version n]
or
        ntp peer <host> [version n]

depending on whether you want a client/server or peer relationship.
There's a bunch of other stuff available for MD5 authentication,
broadcast, access control, etc.  You can also use the
context-sensitive help feature to puzzle it out; try ``ntp ?'' in
config mode.

You'll also want to play with the SHOW NTP * router commands.  Here
are two examples.

EXAMPLE 1:

router# show ntp assoc

      address         ref clock     st  when  poll reach  delay  offset    disp
+~128.9.2.129      .WWVB.            1   109   512  377    97.8   -2.69    26.7
*~132.249.16.1     .GOES.            1   309   512  357    55.4   -1.34    27.5
 * master (synced), # master (unsynced), + selected, - candidate, ~ configured

EXAMPLE 2:

router#show ntp stat
Clock is synchronized, stratum 2, reference is 132.249.16.1
nominal freq is 250.0000 Hz, actual freq is 249.9981 Hz, precision is 2**19
reference time is B1A8852D.B69201EE (12:36:13.713 PDT Tue Jun 14 1994)
clock offset is -1.34 msec, root delay is 55.40 msec
root dispersion is 41.29 msec, peer dispersion is 28.96 msec

For particular cisco NTP questions, feel free to ask in comp.dcom.sys.cisco.

For broader NTP info, see ftp://louie.udel.edu:pub/ntp/.  The file
clock.txt in that directory has info about various public NTP servers.
There is also information on radio time receivers that can be
connected to an NTP server (this is handy on private networks, if you
have an entire campus to get chiming, or if you become a hard core
chimer).

The ``ntp clock-period'' command is added automagically to jump-start
the NTP frequency compensation when the box is rebooted.  This is
essentially a representation of the frequency of the crystal used as
the local timebase, and may take several days to calculate otherwise.
(Do a ``write mem'' after a week or so to save a good value.)

Caveat of obsolecence: Note that the CS-500 will not be able to
achieve quite the same level of accuracy as other platforms, since its
hardware clock resolution is roughly 242Hz instead of the 1MHz
available on other platforms.  In practice this shouldn't matter for
anyone other than true time geeks.