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How can I Prevent Telco Loops on Cisco 2901 T1 Service-Module HWIC-1DSU-T1

I have an issue where one of our T1 circuits is bouncing. It connects to a service-module in a Cisco 2901 with an HWIC-1DSU-T1. We opened a telco ticket and every time they test they seem to leave a "remotely initiated loopback" up on the service-module.  We have to dial in through OOB and drop the loop by doing "clear service-module serialx/x/x."   Is there a way config we can use to prevent telco from looping up our service-module?   We think this last loop was left up for about 4 hours, but we can't tell in the logs for sure because we only see the interface going down.  Is there also a way to cause a log to generate that shows exactly when a loopup code is received by our service-module?

 

sho service-mod serial#####
Interface Serial#####
Module type is T1/fractional
    Hardware revision is 1.0, Software revision is 001,
    Image checksum is 0x8B7398, Protocol revision is 0.1
Receiver has no alarms.
Unit is currently in test mode:
    remote loopback (remotely initiated)
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.
Last module self-test (done at startup): Passed

7 REPLIES 7
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VIP Expert

Hello,

 

usually, you can configure 'no loopback' on either the controller or the interface. However, I would not recommend doing that, since TELCO initiated loopback tests are very common. The real question is what exactly the Telco does to keep the interface from coming back up. That is what you should go after. In the meantime, you can use an EEM script such as the one below to automatically clear the service module in case  the interface goes down:

 

track 1 interface Serial0/0/0 line-protocol

!

event manager applet TRACK_SERIAL_LINE
event track 1 state down
action 1.0 syslog msg "Remotely Initiated Loopback active"
action 2.0 cli command "enable"
action 3.0 cli command "clear service-module serial0/0/0"
action 4.0 cli command "end"
action 5.0 syslog msg "Service Module cleared by EEM Script"

Highlighted

Thank you for your reply. Thank you for the suggested EEM script. If we are not seeing any messages in our logs when they loop up our service-module will this script still work? If not, how can I get the syslogs to show when the loop up or loop down commands are received by the service-module. I know on our other routers that have T1 controllers we can see in the logs when they loop up our CSU. But we do not see those log messages on our routers with service-modules.
Highlighted

Hello,

 

what is the line protocol status when the loopback is active ? Is there anything at all logged for the interface ? You can change the script tp match whatever syslog message is generated (or you can configure an IP SLA)...

Highlighted

As I was reading the original post I was thinking of an EEM script as the solution. I congratulate Georg for suggesting it before I did. I do have one thought about using this script. Would this script have a negative impact on telco attempt to run a genuine loopback test if there were real issues with this circuit?

 

HTH

 

Rick

HTH

Rick
Highlighted

Very good point actually, Richard. I am not sure how long the loopback tests last, I remember them to be fairly short. A delay up/down could be configured for the tracker:

 

track 1 ip sla 1 reachability

delay down 30 up 60
!
ip sla 1
icmp-echo 8.8.8.8 source-interface Serial0/0/0
frequency 10
ip sla schedule 1 life forever start-time now
!
event manager applet TRACK_SERIAL_LINE
event track 1 state down
action 1.0 syslog msg "Remotely Initiated Loopback active"
action 2.0 cli command "enable"
action 3.0 cli command "clear service-module serial0/0/0"
action 4.0 cli command "end"
action 5.0 syslog msg "Service Module cleared by EEM Script"

delay down 30 up 60

Highlighted

Georg

 

I read the original post and was thinking about the EEM script as a solution. The potential impact on telco did not occur to me until I was reading your suggested script. So I might very well have done as you did to suggest an EEM script without anticipating the telco effect. Yes I believe that the delay should give the telco some opportunity to do their testing. Might need to do some tweaking of the timers to optimize telco needed time.

 

HTH

 

Rick

HTH

Rick
Highlighted

Hello,

 

the EEM script in conjunction with the IP SLA would look like this:

 

track 1 ip sla 1 reachability
!
ip sla 1
icmp-echo 8.8.8.8 source-interface Serial0/0/0
frequency 10
ip sla schedule 1 life forever start-time now
!
event manager applet TRACK_SERIAL_LINE
event track 1 state down
action 1.0 syslog msg "Remotely Initiated Loopback active"
action 2.0 cli command "enable"
action 3.0 cli command "clear service-module serial0/0/0"
action 4.0 cli command "end"
action 5.0 syslog msg "Service Module cleared by EEM Script"