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Clock source from different carriers

Frequent Contributor
Frequent Contributor

Below is an extract from one our Voice GW - Based in the UK

network-clock-participate slot 1

network-clock-participate slot 2

network-clock-participate slot 3

network-clock-participate slot 4

network-clock-select 1 E1 1/0/0 PABX A

network-clock-select 2 E1 1/1 SPARE

network-clock-select 3 E1 2/0 PABX B

network-clock-select 4 E1 2/1 Telco carrier A

network-clock-select 5 E1 3/0 PABX B

network-clock-select 6 E1 3/1 SPARE

network-clock-select 7 E1 4/0 PABX B

I`ve been asked to standardise our clocking sources, In the near Future we are also installing a new Telco Link from Carrier B as we removing PABX A so only having Telco Carrier A, B and PABX B

We also have Telco A and B link load balance on other routers

My thinking is to change the above so to Clock from 2/1 as the priority 1 and with the others set to Internal so to clock of 2/1 however if we lose 2/1 we lose Clocking. Can I set the new Carrier B as secondary clock priority with the commends Clock Source Line Primary- Carrier A and Clock Source Line for Carrier B

network-clock-select 1 E1 2/1 Telco carrier A

network-clock-select 2 E1 2/0 Telco carrier B

However I seem to remember this can be an issue in that we swap over to the secondary if we clock from a different source so it can effect the PABX links?

or will it be better to only have the Telco carrier A as first and the PABX B as secondary?

network-clock-select 1 E1 2/1 Telco carrier A

network-clock-select 3 E1 2/0 PABX B

network-clock-select 5 E1 3/0 PABX B

network-clock-select 7 E1 4/0 PABX B

So not have the Telco Carrier B as any standby clock source


2 Replies 2

Felipe Garrido
Cisco Employee
Cisco Employee

Technically, the E1s should be separated into groups so that only E1s from the same source are in the same clocking domain. To do this, all the E1s from PABX B should be installed in either a separate NM slot or on the mainboard HWIC slots. same goes for E1s from the other sources.

DSP sharing would also need to be disabled between the NMs and mainboard.


Hi Felipe

Thanks for the  reply, i`ve since been informed that the Telco`s A&B should clock from same In-Country Source,   Will this make a difference now?

Ideally we would want to load balance the PABX and Telco`s across different cards on the same  router so PABX links are on 2/1 3/0/1 4/1 and  Telco 2/0 3/0/0 3/1 etc rather have them on the same slots- Are you saying all PABX  should in grouped together on same NM or VWICS`s  so PABX 2/0 2/1 2/0/0 2/0/1 while Telco`s are on 3/0 3/1 3/0/0 3/0/1 etc

Telco line will be the primary and the PABX internal clocking from the Primary clock source

Extract from Cisco 

When using a two-port network module, such as a VWIC2-2MFT-T1/E1 for two voice circuits, both ports must use the same clock source. The same DSP resources used for both ports and differences in clocking can cause sips and interface flaps. If multiple different clocks sources are present (such as circuits from different carriers), divide them across modules using different DSP resources. For example, one could use motherboard DSPs and another could use DSPs on a network module (NM), such as the NM-HDV2. In this case, you should not include the NM-HDV2 in the network-clock-participate list. Because the NM has a different clock than the other circuits in this example, it should not be a fallback candidate to clock the TDM bus

How do you  disable  DSP sharing?

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