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New Member

WAN link clocking question

I hope this makes sense:

When you have a WAN link proving data connectivity to your branches, say I have two types of connections:




On the point to point connection, If I am providing both routers in the connection and have the serial interface to serial interface, will there always be one router providing the clocking and the other taking it's clock from the other router, or can the provider supply the clock on the line and both ends get the clock from the line?

same question for MPLS, my current MPLS cloud has my routers set the clock to "line" am I to assume the provider is supplying the clock and he has all of his serial interfaces clocking as "internal"?


Re: WAN link clocking question

You should always set you clock to line on all routers.

You almost never have a piece of wire running between 2 routers there is always some kind of telco equipment between you. It is this equipment that provides the clock not the routers.

Now if you are talking things like oc3 or even some ds3/e3 then there are cases where the router needs to be set to internal but for lower speed lines you almost never use the router to clock the circuit.

New Member

Re: WAN link clocking question

Thanks for the reply.

The telco equipment that sets the clocking, is it their Provider Edge ROUTER setting the clock?

For example, is it as simple as they have their router configured to "clock internal", or is something else providing the clocking on the line?


Re: WAN link clocking question

Now I can only guess since I don't do transport that much.

There is more than likely some kind of mux that is providing it. Nowdays point to point is really just some fraction part of a fiber network. I know some of our providers have cisco ONS 15400 on our premise along with a number of other boxes. Somehow this box will take a number of oc-48 and output just about any type of circuit the vendor wants and it also produces the clocking I am assuming.

Hall of Fame Super Silver

Re: WAN link clocking question

Hello Wilson,

>> The telco equipment that sets the clocking, is it their Provider Edge ROUTER setting the clock?

Usually it isn't , because there are other devices that can stay in the middle like ADM optical ADM, it is the provider edge router only if the two routers are colocated.

Usually the router interfaces are the users of some transport service and act as DTE receiving clock.

Hope to help


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