[ntp:questions] very slow convergence of ntp to correct time.

Brian Utterback brian.utterback at sun.com
Mon Jan 28 21:00:11 UTC 2008

Rick Jones wrote:
> Eric <nospam-01 at jensenresearch.com> wrote:
>> Then there is the MAC cache in your switches, which generally purge
>> after 1-5 minutes.  This can often be adjusted higher, but that can
>> sometimes cause issues for others when they are reconfiguring part
>> of the network.
> I suppose if STP gets involved, but just on its own, the forwarding
> table in a switch being aged should only mean that the next frame to
> that MAC will go out all (enabled) ports on the switch until that MAC
> is seen again as a source.  That shouldn't affect timing really.
> rick jones

There is some blurring of device type going on. The problem is not
which port to send to, but rather which MAC address to send to. This
is more of a problem with routers than switches, but with VLANS and
whatever these days, the device uin question might be both.

In any case, if the needed MAC is not available, there has to be an
ARP request and response before the packet can be sent, but this
delay is not evident in the return trip for the NTP response packet,
introducing an asymmetric delay, the worst thing that happen to

I reported this problem many years ago and suggested using burst
at that time, but thought that it would be overkill and asked
for a way to tune it to a fewer number of packets in the burst.
Dave was reticent and I was newer to the project then and didn't
want to push it. Perhaps it is time.

Brian Utterback

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