[CentOS] Centos machine sometimes unreachable

[Les Mikesell]

On Wed, Aug 22, 2012 at 8:01 AM, Richard Reina <gatorreina at gmail.com> wrote:
> I have a simple perl script that every few hours pings the handful of
> machines on my LAN. Lately I've sometimes been getting
>
> ping of 192.168.0.1 succeeded
> ping of 192.168.0.7 succeeded
> ping of 192.168.0.5 FAILED
> ping of 192.168.0.6 succeeded
> ping of 192.168.0.9 succeeded
>
>   This machine in question has been running Centos faithfully for about six
> years and no recent changes to it have been made. When I try and ping the
> machine manually it works.   /var/og/messages does not seem irregular. Does
> anyone know know what might be the problem or what else I might check?
>

What does ethtool say about your duplex setting, and is the connected
switch set one way or the other?   In the long distant past, cisco
switches weren't very good about negotiating and they recommended
configuring full duplex at both ends of the connections.  Now the
hardware and standards are better and everyone uses auto, but you
might have an old switch or an old configuration somewhere in the
path.   The problem is that if one end of the connection is set to not
negotiate, the other end will pick half duplex and the mismatch will
cause trouble at random.

-- 
   Les Mikesell
    lesmikesell at gmail.com