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

Eric nospam-01 at jensenresearch.com
Mon Jan 28 21:48:01 UTC 2008


On Mon, 28 Jan 2008 21:39:17 +0000 (UTC), Rick Jones <rick.jones2 at hp.com>
wrote for the entire planet to see:

>Eric <nospam-01 at jensenresearch.com> wrote:

>> Of course, different manufacturers may have different methods of
>> detecting the cache miss and recovering from that, so it would be
>> hard to eliminate that effect from consideration entirely.  It's the
>> smallest effect of all the ones I've dealt with.
>
>Just to be certain, you are talking about MAC's being aged out of a
>switch's forwarding tables right?  I interpreted it that way based on
>the previous text discussing ARP caches.

Yup.  And I see that in the simple case the packet just floods, isn't
delayed on its original path/port, and the MAC cache update is handled
overlapped in time with the packet transfer.  

But, there may be more complicated cases; you mentioned STP, and of course
flooding causes its own delays to some degree.  Then it might be that the
switch firmware takes a slow path on the cache miss, causes an interrupt,
gets scheduled into a timeslice, updates the MAC Cache, and then redrives
the packet forwarding process.  Not ideal, if that ever happens.

- Eric




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