CCNP - x25 DCE question

Author

x25 DCE question

peterd

2001-11-02, 7:13 am

Hi everyone,

I picked up some questions off a web-site somewhere, can't remember where now but it was some time ago, and the answer that they give for one question doesn't look right to me...

Q) Pick two correct statements about x25 DCE devices:

a) x25 DCE provides clocking at the physical layer

b) an example of an x25 DCE is a network switch or concentrator

c) the x25 DCE occurs at all four layers associated with the x25 stack

d) these devices provide the interface between x25 DTE and the x25 switched network.

They say the answer is 'a' & 'b'.

To be honest I can't say if either of these are right or wrong (it could go either way...they look about right!), but I do think that 'd' is correct. 'c' looks totally wrong to me and I wouldn't consider it at all. Any ideas?

regards
Peter

Yankee

2001-11-03, 10:05 am

I would think D is wrong because the DCE device is the switch, not an interface device.

Yankee

doctorcisco

2001-11-04, 4:50 am

quote:
Originally posted by peterd
To be honest I can't say if either of these are right or wrong (it could go either way...they look about right!), but I do think that 'd' is correct. 'c' looks totally wrong to me and I wouldn't consider it at all. Any ideas?

IMO, this is a good example of a ridiculous question. I agree that "C" is out (what's a "layer 4 DCE??"), but the rest turn into a debate about words. A, for example ... if I have an X25 T1 or E1, the physical layer clocking could come from any number of devices -- a MUX or perhaps a CSU/DSU at the telco end would be two likely suspects. These are, I suppose, "DCE" in the broad sense of the term, but are not "X25 DCE" devices in the sense they are unique to X25 circuits.

As for "B," yep, it's true ... but refers to the layer *2* DCE, which usually is not the same device that provides layer 1 clocking mentioned in A!

When I see garbage questions like this, I always remember the frame relay switch in my lab. There are two specific commands used that reflect the fact there is layer 1 DCE and a layer 2 DCE, and the two are not to be confused:

1) The DTE and DCE ends of the crossover cable, which tell you which router needs the "clock rate" command on the physical interface. This defines which end is the layer 1 DCE.

2) The "frame-relay intf-type dce" command, which tells the router to reply to LMI queries from the device at the other end (i.e. to be a frame relay switch). This defines the layer 2 DCE.

It is certainly possible to have 1 end be the layer 1 DCE, and the other end be the layer 2 DCE. I'm sure the same sort of thing applies to X25.

The bottom line: IMO, this question mixes up the 2 different kinds of DCE so badly it's not a worthwhile question.

HTH,
doctorcisco

depamo

2001-11-04, 7:31 am

I wasn't aware that Frame Relay had this type of setup but I do know that you can have this happen with X.25 where you delcare a physical layer DCE based on the cabeling and a layer 2 DCE to act as a controller of sorts for managing the link. The two are independent of each other and a physical DCE can be a layer 2 DTE, just as you described with Frame Relay.

peterd

2001-11-05, 3:53 am

Hi Guys,

I'm glad it's not just me that has a problem with the wording on this question.

The point about answer 'd' is that, taken literally, a 'switch or concentrator' (from answer 'b' could be classed as a form of 'interface'. Though not an 'interface device' as Yankee says, but any area where two 'things' come together is an 'interface'.

It's so easy to get confused over words...

and especially in the way that the UK and the US attach slightly different meanings to so many of them!

I feel sorry for those whose first language isn't English as some of the terms used by Cisco (and 'practice' exam writers) confuses the heck out of me at times.

It's down to Poke'n'Hope sometimes!

Regards
Peter

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