Re: Clock Rate

Gaurav,
I would like to share my understanding of what clock is.
I guess it should be clear that clock rate is of no use in
dynamips and its clients (dynagen/GNS3), as no phy layer
emulation takes place there, and data is transferred using
UDP between interfaces.

When you have an asynchronous link, the bits timing is recognized
on each byte boundary in an independent way. Each byte is independent of
the previous, and there is no out of band reference between Tx and Rx.
This is expensive in terms of time, because you need start bits and
stop bits.

So in synchronous links, a reference timing is kept at the endpoints
to be reused for all bits/bytes. DCE's do that (recover the clock)
and share it via an out of band signal with the DTE. Each side of
a serial link is supposed to have a DCE and a DTE.

So routers (DTEs) should actually use clock info from its associated
DCE. Routers has no business in clocking.

But we sometimes use routers connected back to back, and we don't want
to set up DCEs, so we use sort of null modems, thanks to the ability
of routers to fake being a DCE to the linked router. This faking implies
the provision of a clock reference, hence the clock rate command, and
a specially wired cable that connects a clock source pin on the smart
connector to the pin used by DCEs to provide the clock. What we call
a DCE cable. Routers recognize the presence of such cable (that's why
we call them smart cables I guess :) and enable the clocking.

Last, some actually use cables that combine a DTE cable and a DCE
cable, becoming real "null modems". Those cables do have a DCE end
and a DTE end indeed. AFAIK, cisco does not sell such cables, but
they are handly in labs.

HTH,
-Carlos

GAURAV MADAN @ 17/07/2011 11:35 -0300 dixit:
> :)))))
>
> DTE , DCE are devices (routers in your case) each having defined purpose .
>
> But you need to plug in right cable to right device .
> Physically a serial WIC / HWIC card plugged in a router can assume a
> role of a DTE as well as DCE.
> It depends on the cable that is connected to serial interface that the
> end takes a role of DCE or a DTE .
>
> Each serial cable type (RS 232 , V.35 , 449 , 530 etc etc) have a DTE
> end and a DCE end ; so depending on what is plugged in where .. the end
> is defined .
>
> Hope some clarity this time ,
>
> Gaurav Madan
> CCIE .
>
> On Sun, Jul 17, 2011 at 7:05 PM, Carlos G Mendioroz <tron_at_huapi.ba.ar
> <mailto:tron_at_huapi.ba.ar>> wrote:
>
> This is getting so distant from reality ... as a lab can get.
> What exactly is DCE and DTE ? Is a router a DTE or a DCE ?
> Is the cable a DCE or a DTE ? :)
> -Carlos
>
> Radioactive Frog @ 17/07/2011 01:57 -0300 dixit:
>
> Great explaination Gaurav.
> Just would add on top of gaurav's statement - You need a very
> SPECIFIC
> cables that has one end DTE and other end DCE to get it working.
>
> On Sun, Jul 17, 2011 at 2:12 AM, GAURAV MADAN
> <gauravmadan1177_at_gmail.com
> <mailto:gauravmadan1177_at_gmail.com>>__wrote:
>
> Things to consider here are :
>
> 1) Clock rate is applicable to DCE devices and not on DTE
> devices
> 2) Devices can be categorized as DCE or DTE depending the
> cable connected
> to
> serial interface.
> 3) Generally DCE devices are FR switch etc.
> 4) If you are configuring the back to back connection ;
> configure "clock
> rate <> " CLI on DCE end only
> 5) I think latest IOS on serial HWIC will take clock rate
> command only on
> DCE end and not on DTE end . However older IOS used to take
> it on any
> device
> 6) Any rate is good ... as long as it is working .. default
> 9600 bps should
> be OK .
>
>
> HTH
> Gaurav Madan
> CCIE
>
> On Sat, Jul 16, 2011 at 9:36 PM, <jhnrose401_at_gmail.com
> <mailto:jhnrose401_at_gmail.com>> wrote:
>
> Dear
>
> I want to know what clock rate i will use when i am in
> the lab or when i
>
> am
>
> using
> dynamips what is the method or way i will configure
> the correct clock
> rate to
> my routers
>
> Eg
>
> For DCE i ll configure the clock rate not for DTE correct
>
> if both devices are DCE i will define CR on both or in a
> single devices.
>
> R1 --ser1/0 -------------------s1/0 R2
>
> I am facing problem my ospf adj flap after few minutes
> is CR are the
>
> reason
>
> R1 = s1/0 ip add 10.1.1.1/24 <http://10.1.1.1/24> and
> loop back 1.1.1.1/24 <http://1.1.1.1/24>
> R2 = s1/0 ip add 10.1.1.2/24 <http://10.1.1.2/24> and
> loop back 2.1.1.1/24 <http://2.1.1.1/24>
> clock rate 64000 on both sides
> R1 and R2
>
> router ospf 1
> net 0.0.0.0 255.255.255.255 are 0
>
> thks
>
>
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>
>
>
>
>
>
> --
> Carlos G Mendioroz <tron_at_huapi.ba.ar <mailto:tron_at_huapi.ba.ar>>
> LW7 EQI Argentina
>
>

-- 
Carlos G Mendioroz  <tron_at_huapi.ba.ar>  LW7 EQI  Argentina
Blogs and organic groups at http://www.ccie.net