Sujoy's Tech Blog: ISDN PRI Troubleshooting and RJ-48 Ethernet Loopback Test on Cisco Router_Primary Rate Interface Problem Fixed

Hi, I'm Sujoy and today I'll show you how to troubleshoot ISDN Primary Rate Interface (PRI) line and determine working status of ISDN PRI Multiflex Voice WAN Interface Card on Cisco router.

I recently faced a problem where customer can't communicate over the ISDN PRI leased line. I had to do a loopback test on PRI interface to determine if the problem is with ISDN PRI line or PRI card on Cisco 2921 Router. I'll show you how to do step by step troubleshooting.

Step 1- Look at the picture below. It shows VWIC2-1MFT-T1/E1 card on Cisco 2921 router.
VWIC2-1MFT-T1/E1 stands for Voice WAN Interface Card v2 - 1 Port Multiflex Trunk T1 or E1 line card.

Check physical layer status, first look at the card and the interface. If the Ethernet cable is connected and LEDs are glowing then the interface is up. If cable is not connected, connect the cable coming from PRI adaptor and check if interface is up. If not up, check if interface is administratively down and check other configuration.

Step 2- Below is the picture of PRI-to-Ethernet adaptor which converts ISDN line to standard RJ-48 Ethernet line for terminating to Cisco router.

Check the PRI to Ethernet adaptor if any wire got disconnected. If not, we'll move on to Router troubleshooting.

Step 3- Establish a console or telnet session with router and go to Router# prompt (privilege mode). Give command- show inventory and check if the PRI card is showing in inventory. In our case, it's detecting and is highlighted in picture below. If it's showing, then the card is assumed to be ok.

Step 4- Now from Router# prompt give command- show isdn status and check layer 1 status. In our case, layer 1 is SHUTDOWN. If layer 1 (physical layer) is SHUTDOWN, then layer 2 (data link layer) and layer 3 (network layer) will not be able to function.

Layer 1 can be SHUTDOWN due to few reasons- 1) Card is faulty 2) Signaling or keepalives not being received from ISP side 3) wrong configuration 4) Ethernet cable not connected 5) Interface is administratively down. I know reasons 4 and 5 are not valid. I've to identify reasons 1, 2 and 3.

Step 5- Check running-configuration. From Router# prompt give command- show run and gather few information shown in below picture.
card type
controller type
serial interface number
voice-port number

Check if any recent configuration changes have been done. If there are no recent configuration changes done on router before PRI line went down, then it can be issues with PRI card or line. You can check that by command- show run

Router# show run

Building configuration…

Current configuration : 9363 bytes

Last configuration change at 13:54:46 IST Wed Apr 20 2016 by Admin

If no configuration changes are done, then we'll move on to loopback test.

Step 6- Check controller status. From Router# prompt give command- show controller e1 0/0/0 and check status up/down. Notice in line number 7, Framing is NO-CRC4 and Line Code is HDB3. These values are generally ISP provided and cause line down if mismatch. Try changing them and see if this works.
Also check if controller, serial interface, voice port is administratively down. If so, enable them by no shutdown command.

Step 7- If that did not work, we need to do loopback test to identify if the problem is with router & card or ISP/telco PRI line.

For this test, I need a Ethernet Loopback Plug. How to make Ethernet Loopback Plug ? Take any ethernet cable and cut after three inch. Now hold the RJ-45 plug keeping pins below and locate pin 1,4, 2, 5. Peel plastic cover of above pins and short copper wires together. Short pin 1 and 4 together and pin 2 and 5 together. Bend slightly outward to avoid short circuit between this two couplings.

Step 8- Connect the loopback plug to PRI interface. And check if LED is glowing. If not, wiring is wrong.

Step 9- Now for loopback test we'll look at the running-config. We derived it in step 6.
In our case, card type e1 0 0 (means e1 card, slot 0 and subslot 0), controller e1 0/0/0 (same as card type,device/slot/subslot), interface Serial0/0/0:15 (it's virtual serial interface for communication), voice-port 0/0/0:15 (voice ports). Give below commands-

Router#config t

Router(config)#controller e1 0/0/0
Router(config-controller)#no pri-group timeslots 1-31
If the above command is accepted, then ok. Otherwise you've to do few more steps.
Router(config-controller)#no pri-group timeslots 1-31
%controller must be shutdown before unconfiguring pri-group to gracefully bring down isdn.
Router(config-controller)#shutdown

Step 10- Shutting down the voice interface. If it gives an error like the one below-
%voice interface must be shutdown before deleting pri-group.
Then give below commands-
Router(config-controller)#exit
Router(config-controller)#voice-port 0/0/0:15
Router(config-controller)#shutdown

Step 11- Removing layer 3 binding. If it gives an error like the one below-
%Remove layer 3 binding first
Then give below commands-
Router(config-controller)#exit
Router(config-controller)#int Serial0/0/0:15
Router(config-controller)#no isdn bind-l3 ccm-manager

Router(config)#controller e1 0/0/0
Router(config-controller)#no pri-group timeslots 1-31
This time the pri-group command will be accepted. Sometime you may have to give below command to create the serial 0 interface.
Router(config-controller)#channel-group 0 timeslots 1-31

Step 12- Assigning IP address for loopback test.
Router(config-controller)#exit
Router(config)#int Serial0/0/0:0
Router(config-if)#encapsulation hdlc
Router(config-if)#ip add 10.10.10.10 255.255.255.0
Router(config-if)#no shutdown

Step 13- Checking Serial Interface status and enabling controller e1.
Router#show int Serial0/0/0:0
Router#config t
Router(config)#controller e1 0/0/0
Router(config-controller)#no shutdown

Step 14- Doing loopback test.
Router#ping ip 10.10.10.10 repeat 1000
If all 1000 pings are getting reply (!) then your PRI card interface is working fine. The problem is with the line. Contact your ISP/telco for line checkup. You can also check that the show controller e1 0/0/0 shows status up.