Clocking issue on 3800 isr

Unanswered Question
Apr 30th, 2009

Hi All,

I am figuring out how to configure clocking for a 3845 isr with about 10 E1s correctly.

At the moment, the router has 1 clocking domain (all VWICs, NM are participating on this domain).

I configured e1 0/0/0 to be the 1st choice clock source

controller e1 0/0/0

clock source line

network-clock-select 1 e1 0/0/0

I configured e1 0/0/1 to be the 2st choice clock source

controller e1 0/0/1

clock source line

network-clock-select 2 e1 0/0/1

My question is, how does e1 0/0/1 clocking work when 0/0/0 is up and running fine? Is it using clocking signal from the back plan or is it using PSTN as its own clock source and ignore the back plane (for both Tx and Rx)?

Many thanks in advanced.

Btw, I found that cisco documentation around clocking for 3800 isr could not be more difficult to find. If you know any good source of documents, please let me know. thanks.

I have this problem too.
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Nicholas Matthews Thu, 04/30/2009 - 10:08


Each set of DSPs will have it's own clocking source. If all the DSPs are on the motherboards, such as PVDM2-X on an ISR like this, then they will all be on voice-card 0.

In this case the clocking source can either come from an external source (clock source line), or the gateway will create it's own internal clock and trust that.

For clock source internal there is no need to 'select' this clock because it is assumed.

For clock source line there are multiple possible clocking sources, because the router has no way of knowing whether or not all of the lines are on the same clocking domain. For this reason there is the 'network-clock-select' command. It will tell the router which of the external clocks to send over the backplane.

When your selected clock is active, this clock is sent over the backplane and the other DSPs will use this. If you're in a situation where you have multiple clocking domains, all the clocks that match the internal clock, which would be an external clock, will be fine. But the external clocks that do not match the selected clocks with probably have slips and errors.

If you're in a situation where there are mutliple clocks, the solution is to get something like an NM-HDV card where those DSPs would be able to provider a new clock on the 'backplane' of the card.



thuchien0308 Fri, 05/01/2009 - 03:00

Thanks Nick,

All your points are valid and helped.

In my case the e1 0/0/0 and e1 0/0/1 are connecting to different PSTN provider (i.e. different clock sources)

So as you said, there is no way to avoid clock slips on the 2nd E1 0/0/1 ?

Would command like "clock source line independent" actually help on e1 0/0/1 to operate as independant clock domain (both Tx and Rx used same clock source from line) while 0/0/0 is still up? and when e1 0/0/0 goes down, will e1 0/0/1 become primary clock source for the tdm backplane without issues?

Thanks in advanced,


thuchien0308 Fri, 05/01/2009 - 05:06


"clock source line primary" is actually useful. However, there is some confusion with this command:

According to cisco the "primary" does:

"Specifies that the PLL on this controller derives its clocking from the external source to which the controller is connected. This option also puts a second port, which is generally connected to the PBX, into looped-time mode. Both ports are configured with line, but only the port connected to the external source is configured with primary."

I am not sure the "second port", it meant is just the second port on the same vwic, or all other "clock source line" ports on all other vwics on motherboard (because all vwics on motherboard have to "network-clock-participate", there is no such thing as PLL for just a vwic).

Could you or anyone help clarify?

Many thanks in advanced.

Paolo Bevilacqua Sat, 05/02/2009 - 10:37

The option is confusing also due the variety of HW and configuration that you can have (VWIC, VWIC2), and the different platforms as well.

Suggest trying the different modes and compare, I don't think you can get better help from the documentation.


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