Configuring Cisco WIC-1DSU-T1-V2 on back to back routers

[Cooky]

I'm trying to set up a back to back T1 connection between two 2800's, and have some questions:

It seems one side's supposed to be configured as the clock source, and the other side to follow it.

However, when I leave everything as default (clock source line), the link is up/up, and I can ping across the cross-over T1 link between the two routers.

Why is it working when the connection isn't yet configured properly?

R2#sh service-module s0/2/0
Interface Serial0/2/0
...
Framing is ESF, Line Code is B8ZS, Current clock source is line,
Fraction has 24 timeslots (64 Kbits/sec each), Net bandwidth is 1536 Kbits/sec.
---

R5#sh service-module s0/2/0
...
Framing is ESF, Line Code is B8ZS, Current clock source is line,
Fraction has 24 timeslots (64 Kbits/sec each), Net bandwidth is 1536 Kbits/sec.

R5#ping 150.100.25.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 150.100.25.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
==========

I've seen on some older routers where the clock & framing, etc are configured under "controller t1".

When would you configure these settings under controller, and when would you do it under service-module?
==========

Also, why are the T1's generally believed to have 1.544M of bandwidth, when it's actually 1.536M? (24 * 64K)

 

[Cooky]

How does the router know which side is DCE, and which side is DTE?
It looks like it's pre-determined...how do I change it?

R2(config)#int s0/2/0
R2(config-if)#service-module t1 clock source internal
R2(config-if)#clock rate 128000
This command applies only to DCE interfaces

 

[xSauronx]

as for which is dce/dte:
i thought that was based on which end of the cable is plugged into which router...guessing from memory, never played with those cables in the labs i did, but im relatively sure one end of the cable determines the DTE, the other end determines the DCE

as for why it works before you have the clock rate set anyway.... *shrug*

 

[spidey07]

Serial can be dce, but not your t1.

If you pushed a lot of traffic you'd see clock slips and errors.

 

[Cooky]

I made my own cross-over T1 cables (1-2-4-5 to 4-5-1-2).
How the heck do I know which end is DCE, and which end is DTE?
Also how did the router know which one's which?

 

[imagoon]

I made my own cross-over T1 cables (1-2-4-5 to 4-5-1-2).
How the heck do I know which end is DCE, and which end is DTE?
Also how did the router know which one's which?

Like spidey said, there is no such thing as DCE / DTE on T1s. If there was, you would not need a T1 cross over.

 

[spidey07]

I made my own cross-over T1 cables (1-2-4-5 to 4-5-1-2).
How the heck do I know which end is DCE, and which end is DTE?
Also how did the router know which one's which?

They are both DTE. Those interfaces cannot be DCE, that's why you need to cross them. You have async ints, not snyc.

 

[Cooky]

Thanks for the explanation, but how then did the router on the other end accept the "clock rate" command, if the interface is DTE?

R2(config)#int s0/2/0
R2(config-if)#service-module t1 clock source internal
R2(config-if)#clock rate 128000
This command applies only to DCE interfaces

R5(config)#int s0/2/0
R5(config-if)#clock rate 128000
R5(config-if)#

 

[imagoon]

Thanks for the explanation, but how then did the router on the other end accept the "clock rate" command, if the interface is DTE?

R2(config)#int s0/2/0
R2(config-if)#service-module t1 clock source internal
R2(config-if)#clock rate 128000
This command applies only to DCE interfaces

R5(config)#int s0/2/0
R5(config-if)#clock rate 128000
R5(config-if)#

It has been awhile but I don't think "clock rate" in the serial interface actually does anything with the T1 service module installed. It is a left over from the old x.35 cables. Generally you left in "unconfigured" because the default rate was correct. If you were doing x.35 cross overs or the like then you used it.

The service module either provides or doesn't provide the clock based on the clock internal / clock line command. T1 cannot be changed as it is a set bitrate.

PS if you trying to test fractional T1 I believe you need to use the timeslots command and tell it which channels are valid.

 

[sactwnguy]

I dont believe that is right, the line still needs a clock to avoid timing slips, you generally don't need to provide it because it is provided by the telco. You only want one device to provide timing on a link. DTE/DCE are just terms used to determine how the interface is pinned out. If two interfaces being connected are the same type the pairs will need to be crossed over. On a back to back router configuration it does not matter which one is providing the clock.

 

[imagoon]

I dont believe that is right, the line still needs a clock to avoid timing slips, you generally don't need to provide it because it is provided by the telco. You only want one device to provide timing on a link. DTE/DCE are just terms used to determine how the interface is pinned out. If two interfaces being connected are the same type the pairs will need to be crossed over. On a back to back router configuration it does not matter which one is providing the clock.

There are 2 clocks. The older clock rate in the serial interface is for the serial ports (x.35 style) If you had a fractional T1 you might configure the CDU/DSU box (not seen much anymore) for 128000 and the serial interface on the router for 128000. The service module has it's own clock (and CSU/DSU built in to the card) and can be told to provide T1 clock or not. However you cannot change the T1 clock as that is set in the T1 standard. Even if you only get a 2 channel fractional T1 you get full T1 timing with the other 21 channels nothing on them. You then use the service module timeslots command to chose which 2 of the channels the 2 channel fractional is on.

 

[sactwnguy]

Im not sure if we are talking about the same thing, the clock on a T1 line is independent of the number of channels. What im talking about is the T1 clock that provides timing on the circuit that is used to synchronize when to look at incoming bits. In a back to back router configuration one router has to provide clock on the line or you will take errors. here is Cisco's doc on the configuration.

http://www.cisco.com/en/US/tech/tk713/tk628/technologies_configuration_example09186a00800a6b82.shtml

and if you want a general overview on how T1 timing works check this out.

http://oreilly.com/catalog/t1survival/chapter/ch05.html

 

[imagoon]

Im not sure if we are talking about the same thing, the clock on a T1 line is independent of the number of channels. What im talking about is the T1 clock that provides timing on the circuit that is used to synchronize when to look at incoming bits. In a back to back router configuration one router has to provide clock on the line or you will take errors. here is Cisco's doc on the configuration.

http://www.cisco.com/en/US/tech/tk713/tk628/technologies_configuration_example09186a00800a6b82.shtml

and if you want a general overview on how T1 timing works check this out.

http://oreilly.com/catalog/t1survival/chapter/ch05.html

This is what I am talking about

R2(config)#int s0/2/0
R2(config-if)#service-module t1 clock source internal This is the T1 clock on the service module. Clock source internal means the unit is using internal timing for the T1 (and providing it as well). The other router should be clock source line.
R2(config-if)#clock rate 128000
This command applies only to DCE interfaces

R5(config)#int s0/2/0
R5(config-if)#clock rate 128000 This is the older x.35 serial configuration. The service module does not use this information.
R5(config-if)#

T1 clock is generated and used only by the service module. Serial configuration that was for the older x.35 + CSU DSU configurations are ignored by the service module.

Like I said above: T1 clock is locked per the spec, It cannot be changed. You can choose the source and the sinks (master clock / slave clock if you will) but you cannot change it. The "Clock rate" command in the serial interface is only for older x.35 networks where the CSU / DSU was a separate box and the CSU/DSU could set it's x.35 port for slower speeds. The CSU/DSU would always run with T1 timings but in a fractional t1 would often only export the combined channel speeds on the x.35 port. So a 2 channel fractional would be exported on the x.35 @ 128k and the router would need the clock speed set to match. If the OP was trying to simulate a fractional he needs to use the service modules timeslots command and choose channels to send data on.