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Community Member

CRC on E1 PRI port

I have a Cisco 3745 voice gateway,with two dual port E1 PRI cards.On card 1 port 1 and card 2 port 1 PRI from Telco 1 are connected.On card 1 port 2 and card 2 port 2 PRI from Telco 2 are connected.

There are high increasing CRC errors, on port 2 of each of the cards,which is effecting the voice quality in VOIP environment.

It seems it is a syncronisation/clocking problem.

I can also see line cord voilation,path code voilation,errored seconds on ---sho controller outport fort port 2 output of each of the dual port cards.

I went to the Cisco site , in which it was explained that first port of each dual port card should have --clock source line primary-- and second port of same card should draw clock from the first port.

But it is not very clear.

The line has been checked from Telco and they have no problem on the Line.

Pls go through the attched attachmetn for config details.

Can somebody tell me if my config requires some modification to solve this problem.

Cisco Employee

Re: CRC on E1 PRI port


The problem that you are experiencing with the clock slips is due to the fact that the two telcos you are connecting to have different quality (stratum) clocks. When connecting a single router with multiple telco's you have to pay very close attention to clocking.

The link below has various examples and solutions to various clocking scenarios

The easist way to resolve your clocking issue would be to move your E1's around so that both PRI's from telco one are on E1 1/0 and 1/1 and the two PRI's from the second telco are on E1 2/0 and 2/1 respectively.

Then set E1 1/0 and E1 2/0 to be clock source line primary and E1 1/1 and 2/1 to be clock source internal.

What this will do is to derive the clock from telco 1 and drive the internall PLL chip on slot 1 with the clock from telco 1. E1 1/1 will then use this clock for its communication. Since E1 1/1 is also connecting to telco 1, the clock will be the same and you will no longer have any slips.

The same concept applies to the E1's in slot 2

Hope that helps.



Re: CRC on E1 PRI port

Hi Ademola,

thanks for the input.

If I shift the cards from the same Telco on same card,I will lose the redundancy i.e. if this card fails, both my lines will go down,

Is there any way maintaining the current physical connectivity i.e. cards from same Telco on the two different dual cards,, we can solve this syncronisation/slips problem

Community Member

Re: CRC on E1 PRI port

Are the second ports on the cards ment to be set to non-crc4 ?


Re: CRC on E1 PRI port

yes they are.

card 1 port 1 telco 1-->crc4 is on

card 1 port 2 telco 2-->crc4 is off(framing no-crc4)

card 2 port 1 telco 1-->crc4 is on

card 2 port 2 telco 2-->crc4 is off(framing no-crc4)

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