Here's the conclusion, since a few people have wondered about it. By the way, the question had to do with an external CSU/DSU.
According to Cisco documentation, a down/down serial interface means the router interface cannot determine that CD has been asserted. As mentioned earlier, a CSU/DSU acts more like a DSU than a CSU and presents a synchronous signal to the router. The CSU/DSU must assert Data Carrier Detect (DCD or CD), Data Set Ready (DSR), and Clear to Send (CTS). The router, which is playing the DTE role, must assert Data Terminal Ready (DTR) and Request to Send (RTS). For all these control leads to be asserted, and for the router to recognize that they are asserted, the router and CSU/DSU hardware must be operational, and the cabling must be properly constructed. However, even if all hardware checks out, there are still cases when the interface may be down/down or flapping between up/up, up/down, and down/down. For example, some CSU/DSUs won't assert CD if the link to the carrier is having problems. In these cases, check the CSU/DSU configuration. Make sure the right clocking, framing, and encoding are configured. If they are correctly configured, then call the service provider and work with the provider to isolate the problem. Priscilla At 03:42 PM 6/22/02, Kevin Cullimore wrote: >Some wrap-up comments: > >Challenged by the incompleteness of the cisco-press materials, I attempted >to produce a complete taxonomy mapping configurations/conditions to >interface/line up/down states. An unimaginably short attention-span cut that >short, but I realized that maybe a superset of that information, up/down >states AND the serial indicators shown at the bottom of the show interface >command. That project ended similarly, but I believe that working in the >other direction might be easier, wherein you tackle the task of categorizing >the large set of circumstances responsible for the four interface states by >examining the entirety of the output for both interface & controller show >commands per misconfiguration, and gradually developing a profile for the >entirety of the show output, allowing patterns to emerege that might render >the first line of the show interface command somewhat intelligible. > >A real possibility is that the case of the integrated csu/dsu serial >interface might have to be treated seperately from the the one involving a >mere serial interface, due to the lack of interaction with an external >device for those functions. > >The two references that dramatically increased my understanding of layer 1 >operations are the paradyne 3160 manual and uyless s black's Physical Layer >Interfaces and Protocols book. > > >----- Original Message ----- >From: "Priscilla Oppenheimer" >To: >Sent: 21 June 2002 8:02 pm >Subject: Re: serial interface down/down or up/down [7:47101] > > > > At 07:17 PM 6/21/02, Chuck wrote: > > >""Priscilla Oppenheimer"" wrote in message > > >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > > > > I guess the question is too hard for a practice test if NOBODY can >answer > > > > it!? > > > > > > > > > >CL: surprise surprise we all THINK we know the answer byt realy we don't >;-> > > > > My messages arrived asynchronously, when I meant for them to arrive > > isochronously. ;-) Seriously, they arrived out of order. Despite all the > > confusion, I do feel like I have an answer now. Thanks to everyone who > > answered. > > > > Priscilla > > > > > > > > > > Here's the thing: Cisco says that a down/down interface means the >router > > > > interface is not sensing a Carrier Detect signal (that is, the CD is >not > > > > active). > > > > > > > > Now, from my studies of V.35 I know that data carrier detect (DCD or >CD) > > > > comes from the DCE side of the V.35 link, carried on pin 8, yadda, >yadda. > > > > It comes from the data interface on the DSU side of the CSU/DSU. > > > > > > > > If the router is correctly connected to the CSU/DSU, will it see CD or > > >does > > > > the answer depend on whether the CSU/DSU is also correctly talking to >the > > > > telco? > > > > > > > > Does "carrier detect" mean literally what it sounds like it means? >Would > > > > the CSU/DSU not assert CD if there was a problem on the telco side? >And > > > > hence the router wouldn't see CD and would say the interface was > > >down/down. > > > > > > > > > >CL: in my old dial up to the BBS days, I always interpreted "carrier" as > > >essentially a completed telephone call. I've carried that "idea" into the >T1 > > >world I live in now. however, I think we all know the problem with Frame >or > > >ATM, where the line is up ( i.e. there is carrier ) because of LMI or the > > >ATM equivalent signalling, so dial backup doesn't kick in, even though >end > > >to end is down. > > > > > > > > > > Not something I can easily test. Maybe I better simplify the question. > > ;-) > > > > > > > > > >CL: personally, I enjoy getting into the nitty gritty of how things >really > > >work. Not that I need to, or that it is necessary, even in the >certification > > >quest. It just serves to broaden the perspective. It never hurts to > > >understand things a bit better. > > > > > > > > > > Priscilla > > > > > > > > At 06:34 PM 6/20/02, Priscilla Oppenheimer wrote: > > > > >Hi Group Study, > > > > > > > > > >While writing some questions for a practice test, I found myself > > > > >questioning what I thought was the right answer. Here's the scenario: > > > > > > > > > >A Cisco router serial interface is correctly connected with a good >V.35 > > > > >cable to the data port on the DSU side of a CSU/DSU. The CSU/DSU has > > been > > > > >misconfigured for the framing method (SF instead of ESF). The framing > > > > >doesn't match what the provider is using. (The question refers to a > > >CSU/DSU > > > > >that is external to the router, not one that is built into the >router.) > > > > > > > > > >Will the Cisco router serial interface be down/down or up/down? > > > > > > > > > >And, would the answer be any different if the question has to do with > > > > >misconfiguring the encoding (AMI versus B8ZS)? > > > > > > > > > >If you have real-world experience with this, that would help. I have > > read > > > > >the Cisco documentation and the troubleshooting charts, etc. > > > > > > > > > >Thanks > > > > > > > > > >Priscilla > > > > > > > > > >________________________ > > > > > > > > > >Priscilla Oppenheimer > > > > >http://www.priscilla.com > > > > ________________________ > > > > > > > > Priscilla Oppenheimer > > > > http://www.priscilla.com > > ________________________ > > > > Priscilla Oppenheimer > > http://www.priscilla.com ________________________ Priscilla Oppenheimer http://www.priscilla.com Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=47207&t=47101 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
