Jumbo Frames and NTP?
The network here has been ready for Jumbo frames on our GigE connections for years – many years. Network performance was always fine, so I never bothered to configure jumbo sized ethernet frames.
Until last week.
Last week I was helping someone on the Ubuntu Forums with their /etc/network/interface file … the mtu setting, specifically. Seemed easy enough, so I manually changed the setting through ifconfig – saw it worked and wanted something more permanent. Modified lots of interface files … some with multiple bridges … got it all working where it could and rebooted. Tested connectivity between GigE and 100baseT boxes everything seemed to work. The GigE connections were not faster – in fact, some were slower by 50% … still 300Mbps, so not THAT slow. Going back or troubleshooting the speed issue would wait until the following weekend maintenance.
During the week, I noticed that many of the system times were off – no – all of them were off by almost 2 minutes. I run an internal NTP server – correct time on my systems is an obsession – has been since the mid-1990s when I learned how important that was for good network security.
I expect Windows systems to drift. It has been an issue for over a decade and Microsoft doesn’t seem able to fix it. Here, the Windows systems sync time every 15 minutes to the internal time server. Every hour was NOT often enough to keep accurate time.
The Linux systems point to the internal NTP server too and keep very accurate time. I’ve never seen them off unless away from the network or I needed to change the timezone while on travel. When I came back from Asia the last trip, it took me over a month to notice the TZ was wrong. The time was very correct, of course, just the TZ was off.
So – the only thing changed last week was Jumbo frames. THAT’s IT. Yesterday morning, I removed the jumbo frames across all the systems and rebooted. Manually forced the Windows machines to get correct times and checked the Linux machines … they’d already corrected the out-of-sync issue on their own. Correct and sub-second accurate again.
Life is good again – time is correct.
Google didn’t find anyone else with jumbo frames interfering with NTP, time. I found that strange. With jumbo frames enabled, the rdate command failed both internally AND externally. That was enough proof for me.
