Serial clockrate question 2

[falinxcopernicus]

I am nearly ready for my ccna exam, but I am confused about 1 thing. In the testking tests, whenever the routers have to be configured, they mention that the clockrate is provided on the serial0 interface, and in the answer if the serial serial0 interface is configured the clockrate is supplied. If however the serial1 interface is configured the clockrate is not supplied. I was always under the assumption that when configuring the comms between two routers both serials should be configured with the clockrate and not just one of the ports?

 

[jdel]

no, only one of the serial interfaces has a clockrate set.

 

[thek25]

I cant remember on any cisco exam where I had to apply a clock rate to an interface, it was always supplied. You have to know which interface requires the clock rate and what the command looks like, etc...How this applies to real world? Most providers will control the clock rate from their end and the individuals purchasing the link 'pipe' to the outside world will not configure it. I've applied clock rate to both sides of the same link in a lab and the only thing I remember about the event was "Hmm, that doenst look right" as the link goes updownupdown

thek25
CCNP, CISSP in progress...

 

[techkiwi]

There is one basic rule to remember, and that is that for serial synchronous connections, you must have one and only one clock.

As thek25 says, in the real world your telco or comms provider will generally supply the clock. For example, if you have a X21 or V35 cable plugged into a CSU/DSU, the CSU/DSU will be providing a clock (which it is deriving from the telco's master clock) on specific pins on the interface to the router. In the lab, if you have two routers connected back to back, you must provide a clock somehow, and generally this is done by configuring one and only one of the serial interfaces to provide clocking.

If you have two sources of clocking (such as the serial interface on both routers), you will have problems. This is because no matter how accurate the clocks are, eventually they will drift and get out of sync with each other, and the end result is data corruption.

HTH

Graham

 

[dcss00]

DTE and DCE, the DCE (CSU/DSU) usually always provides the clock.

But if you have to connect routers in a back to back lab setup one side needs to be a DCE so using the clockrate command will turn the serial port into a DCE and provide a clock.

 

[geek911]

u guys are giving this guy a hard way to answer his confusion. look mister if u have to do that, the testking are just using the s0 as the DCE which usu requires clocking.
if u are not sure which is the dce for now apply clocking to both sides and the router will responds accordingly with either an up line protocol message or an error stating that only applies to DCE.
goog luck

 

[chieftan999]

I agree with geek911.

Only the DCE needs to have the clockrate set. Simple rule, just remember the DCE end.

 

[AdmanOK]

Correct... only the DCE end needs a clockrate command... in fact if you put a clockrate into the DTE end the router won't save it anyway... simply way to remember it is DCE has a C in it and clock starts with C.

 

[binaryops]

you can issue a sh controller s0 command to find which end is the DCE to provide the clock rate.