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New Member

Confused with show commands and router config

Hello Experts - Firstly thanks for the patience involved in answering my questions. I've only recently started working with our 3825 gateway and I am hoping to gain more understanding, and my coworkers are not able to answer my questions. I've studied the command reference guides, but they don't really explain the why/differences in what I see, just the expected output. I've attached a file with the output of 3 commands and have the following questions:

show int

show controllers

show controllers t1

1. Regarding "show int" why are these (Serial0/1/0:0 - Serial0/1/0:22 is down) reported down but Serial0/1/0:23 is up, shouldn't they ALL be "up"? Don't they take calls / data?

and also, why isn't listing for 0/1/1 the same? Instead I see Serial0/1/1:1 is up, but no details for Serial0/1/1:0 - Serial0/1/1:22. What's the difference?

2. for "show controllers" I get only listings for GigabitEthernet0/0, GigabitEthernet0/1, Interface Serial0/1/0:23, Interface Serial0/1/1:1, Interface Serial0/2/0:23.

two questions - why no details for the individual ds0's like in "show int" (Serial0/1/0:0 - Serial0/1/0:22)? These are not considered "controllers" I can presume? So then, can you please articulate the difference between a controller and an interface?

3. Finally, only when I issue "show controllers t1" do I see a listing for T1 0/2/1, why did this not show up in "show controllers command?

Thanks so much for your help.


  • WAN Routing and Switching
Hall of Fame Super Silver

Re: Confused with show commands and router config

Hello Kelly,

1) your router is using the two different T1 links T1 0/1/0 and T1 0/1/1 in two different ways.

T1 0/1/0 is used as an ISDN PRI = primary rate interface.

In this case ser0/1/0:23 is the D channel used for Q.921 and Q.931 signalling to the ISDN switch.

ser0/1/0:1-22 are the bearer channels that are used only on demand for outgoing or incoming voice calls.

You can see some of them up during calls

see description Local-PRI in sh controller T1 output

Instead T1 0/1/1 is used as a data link: some channel B DS0 are grouped in group 1.

The result is a single logical object named serial0/1/1:1 they are seen as a single pipe.

see description T1-LINK-TO-WDC in sh contr T1.


if a T1 or E1 controller is left unconfigured as probably is t1 0/2/1 no logical object is created for it.

As explained above on primary ISDN links the bearer channels are less meaningful then the D channel (:23) so they are not considered in these outputs.

A controller is more related to an hardware, here actually the only controllers of real interest are the T1.

An interface can be a logical object that depending on the underlaying controller configuration can have different meanings:

data link: ser0/1/1:1 it is the whole pipe made of all or part of the B channels

ISDN PRI: ser0/1/0:23 is the signalling channel used for outband signaling to PSTN. B channels are slaves used with a circuit switching logic (one B channel for each voice conversation, multiple B channels if using ISDN for data)

3) as stated above your T1 0/2/1 is probably unconfigured and down

Hope to help


New Member

Re: Confused with show commands and router config

Thank you very much Giuseppe!

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