[ntp:questions] NTP and NAT

Svein Skogen svein at d80.iso100.no
Thu Nov 8 13:39:55 UTC 2007


Terje Mathisen wrote:
*snip*

> Except that the NAT rule traversal is _much_ higher priority/faster than 
> the loacl NTP timestamp. :-(

I agree that this tends to be the symptom, on routers that A) have a
high cpu load, and B) have dedicated ASICs for forwarding packets. It's
not a matter of core priorities, but a matter of the packet forwarding
being run in what for all effects can be considered a co-processor. On
smaller routers (which regularly are used in cases such as this with
only one external IP), with software packet forwarding instead of ASICs,
the NAT process adds a random delay. From what I've seen, what mostly
compromises IOS's ntpd, is cheap interface cards. Adding a single
channelized serial card usually solves the problem. (Don't ask me why,
I'd rather be cleaning Versatec plotters than trying to explain this...)

However, for all intents and purposes, I agree with the symptom of
"packet forwarding having higher priority", since most decent Cisco kit
offloads this to dedicated circuitry, and given Cisco's habit of
underpowering their main cpu on any piece of kit that has dedicated
ASICs, it seems that everything except shoveling packets takes the back
seat.

//Svein

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