Troubleshooting the Physical Layer

The physical layer involves various components, but your main focus is the local loop wiring and clocking. If you have a physical loop, there is clocking, which refers to both timing and synchronization. It is important to understand what's required by the local exchange carrier (LEC) to provide a local loop to the user. Two wires from the line card in the ISDN switch go to a bundle of pairs in the central office (CO) and might split many times into different media, conduits, directions, and multiplexers and/or demultiplexers before finally reaching the field cabinet closest to the user's home. From there, another bundle leads to the user's minimum point of entry (MPOE) or single or split Demarcation Point (D-mark), which is usually a box or cabinet outside the house, in the basement, or in any other easily accessible and protected place. This is the D-mark of the provider's responsibility. Sometimes, especially in rural or hard-to-reach areas, the LEC uses passive or active repeaters. From the MPOE, the CPE zone starts and includes internal wiring (IW) and outlets, such as RJ-11 and RJ-45 connectors. The set of devices and connectors might vary between homes.

To verify the status of the physical layer, it is a good practice to start with the command #show isdn status (see Example 12-1).

Example 12-1. show isdn status Output

 804-isdn#show isdn status Global isdn Switchtype = basic-ni ISDN BRI0 interface         dsl 0, interface ISDN Switchtype = basic-ni     Layer 1 Status:         DEACTIVATED     Layer 2 Status:         Layer 2 NOT Activated 

This command gives you a snapshot of all the layers of the ISDN service. If the first layer is down, the ISDN service is not available. If you are connected to a U interface, the problem is most likely due to the LEC, NT1, or the cable. First, make sure that you are using the proper cable (always check the documentation) from the router to the wall. Look at the front of the router and if the NT1 light is off, the line is down.

The same information is provided on the 700 series routers, as shown in Example 12-2.

Example 12-2. show status Output on the 700 Series Routers

 776-isdn>show status Status    09/01/2000 09:34:30 Line Status   Line DeActivated   Terminal Identifier Unassigned  Not Initialized   Terminal Identifier Unassigned  Not Initialized Port Status                                           Interface Connection Link   Ch:  1      Waiting for Call   Ch:  2      Waiting for Call 

Both routers are reporting Layer 1 Status DEACTIVATED or Line DeActivated. The local loop appears to not be available, not installed correctly, or nonexistent.

One possible troubleshooting tool is to check the status of the Basic Rate Interface (BRI) controller of the router. The commands to do this, and the resulting output, are displayed in Example 12-3.

Example 12-3. show controllers bri0 Output

 804-isdn#show controllers bri0 BRI unit 0:BRI unit 0 with U interface and POTS: Layer 1 internal state is DEACTIVATED Layer 1 U interface is DEACTIVATED. ISDN Line Information:     Current EOC commands:         RTN - Return to normal     Received overhead bits:         AIB=1, UOA=1, SCO=1, DEA=1, ACT=1, M50=1, M51=1, M60=1, FEBE=1     Errors:  [FEBE]=0, [NEBE]=41     Errors:  [Superframe Sync Loss]=2, [IDL2 Data Transparency Loss]=0              [M4 ACT 1 -> 0]=0, 

Both Layer 1 internal state of the BRI controller and the Layer 1 U interface are deactivated. Another important point is to check the status of the Superframe errors. This value always needs to be 0. In this case, the router is reporting 2 Superframe Sync Losses.

The next logical step is to determine if the LEC is not providing the service or if your equipment is malfunctioning. One sophisticated IOS debug command is recommended to answer the question:

 804-isdn#debug bri 

This debug command provides you with information that helps you distinguish between internal and external router problems. This command is the interface between IOS and the ISDN chipset in the router. The output of this command varies between versions of IOS. In some instances, write_sid is sent to the ISDN chipset, which always indicates that the router is trying to bring up the physical layer and set the activation bit (A bit) to 1. At the same time, write_sid instructs the ISDN chipset to generate HDLC flags. Different types of Cisco routers display values such as wrote = E, or wrote = 1B. These values indicate that the ISDN chipset sends HDLC flags. If you cannot see these flags were generated, obviously, you have an internal chipset problem with the router. The sequence of write_sid and wrote=xx, followed by interrupt status reg = C (sent by the chipset to IOS), indicates that the activation bit just turned from 0 to 1. If you type the #show isdn status command again, you see LAYER 1 ACTIVE.^[1] As previously mentioned, these indicators might vary from one IOS version or device to another. As shown in Example 12-4, the IOS version 12.0 5(T) for Cisco 804 and Cisco 1604, reports the following sequence.

Example 12-4. In IOS Version 12.0 5(T), This Is What You See During the Activation of Layer 1

 804-isdn#debug bri *Apr 17 03:13:48.567: BRI0: MC145572 IRQ3: NR1=1 *Apr 17 03:13:48.571: BRI0:MC145572 state handler current state 0 actions     1000 next state 2 ! The MC145572 (chipset) sends NR1=1 to CO *Apr 17 03:13:51.383: BRI0: MC145572 IRQ3: NR1=B *Apr 17 03:13:51.383: BRI0:MC145572 state handler current state 2 actions     2000 next state 2 ! The MC145572 (chipset) sends NR1=B to CO - second attempt *Apr 17 03:13:51.395: BRI0: MC145572 IRQ2: r6=FFF *Apr 17 03:13:51.399: BRI0:MC145572 state handler current state 2 actions     A001 next state 3 ... *Apr 17 03:13:55.559: BRI0: MC145572 IRQ1: m4 bits=FF ! Request to set the higher bit to 1 - (A=1) *Apr 17 03:13:55.563: BRI0:MC145572 state handler current state 3 actions     A001 next state 3 ... *Apr 17 03:13:55.563: BRI0:M4: received 0xFF - changed bits ACT ! The switch sends confirmation of A=1 and as a result ! the status is set to ACTIVE *Apr 17 03:13:55.975: BRI0: MC145572 IRQ2: r6=F00 *Apr 17 03:13:55.975: BRI0:EOC HOLD 

Example 12-5 shows the same procedure for IOS 12.1(4) and a 1604 router.

Example 12-5. IOS 12.1(4) and a 1604 Router

 1604-isdn#debug bri *Mar  1 01:50:25.629: BRI0:sending C/I AI:Activation Indication ! Sending an activation request *Mar  1 01:50:25.633: BRI(0):ext ST new state 1 *Mar  1 01:50:25.637: BRI(0):C/I to 2081 AR:Activation Request *Mar  1 01:50:25.637: BRI0:Unexpected end of mon received data in state 1 *Mar  1 01:50:25.641: BRI(0):C/I from 2081 AR:Activation Request *Mar  1 01:50:25.645: BRI(0):C/I to 2081 AR:Activation Request ! Activation requests exchanged ... *Mar  1 01:50:31.637: BRI0:S2091 state handler current state 2 actions 1083   next state 3 *Mar  1 01:50:31.637: BRI0:receiving C/I AI:Activation Indication ! Here is the indication from the switch for the A=1 setting  line ACTIVE. *Mar  1 01:50:31.641: BRI0:sending C/I AI:Activation Indication *Mar  1 01:50:31.645: BRI(0):C/I to 2081 AR:Activation Request *Mar  1 01:50:31.664: dchan tx start *Mar  1 01:50:31.668: dchan tx intr 

Both of the examples distinguish internal from external issues. If the router sends requests to establish the activation bit and a response is not received, the problem is external, such as the cabling, LEC, or NT1. In this case, your recommended action is to open a trouble ticket with your local provider and fix the issue.

If no request is made from the router, continue to analyze it because the problem might be related to a hardware failure or an IOS bug. One possible solution might be to power-cycle the router in order to force the router to resync with the ISDN switch and reset the BRI U MLT timers (TPULSE and T75S). Usually, the IOS-based timer reset is more typical for the PRIs than BRIs because of the simplicity of the latter.

Another possible solution is to clear the D channel with the IOS command #clear interface bri0, which resets the BRI controller.

You might want to flush the D channel and see if you can get the D channel to work. The interface BRI0 configuration command is isdn timeout-signaling.

If none of these solutions work, consider another CPE.

What you are trying to achieve is the activation of the physical layer (Layer 1). The expected result from the #show isdn status command is displayed in Example 12-6.

Example 12-6. show isdn status Output

 804-isdn#show isdn status Global ISDN Switchtype = basic-5ess ISDN BRI0 interface         dsl 0, interface ISDN Switchtype = basic-ni     Layer 1 Status:         ACTIVE ! As you can see the line is reported ACTIVE <output omitted> 

In the case of a 77x router, the report is as displayed in Example 12-7.

Example 12-7. show status Output

 776-isdn> show status Status    01/05/1995 02:36:52 Line Status   Line Activated ! Here the line is reported as a "Activated" 

Typically, to find out if Layer 1 is active, an engineer checks the status of interfaces. If you apply the same approach here, you see the information displayed in Example 12-8.

Example 12-8. show interfaces bri 0 Output

 804-isdn#show interfaces bri 0 BRI0 is up (spoofing), line protocol is up (spoofing)   Hardware is BRI with U interface and POTS   MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec,      reliability 255/255, txload 1/255, rxload 1/255   Encapsulation PPP, loopback not set   DTR is pulsed for 1 seconds on reset   Last input 00:01:26, output 00:01:26, output hang never <output omitted> 

The BRI0 can be up and the line protocol can be spoofing, but you won't be able to make calls out. As mentioned in Chapter 11, "Cisco ISDN Configuration Solutions," spoofing enables the ISDN router to imitate an active state.

The ISDN configuration that follows includes spoofing. However, DDR becomes active only when interesting traffic triggers a router to place a data call. The status of the D channel is shown in Example 12-9.

Example 12-9. show interfaces bri 0 Output

 804-isdn#show interfaces bri 0 BRI0 is up/down, line protocol is up (spoofing/down)   Hardware is BRI with U interface and POTS   MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec,      reliability 255/255, txload 1/255, rxload 1/255   Encapsulation PPP, loopback not set 

Spoofing does not necessarily mean that the D channel is up and it's not an indication of a properly functioning physical layer. In fact, there might be no line on the interface. Spoofing just implies to the Layer 3 DDR that the routing entry will be maintained in the router, enabling DDR to wake up and make a call out when interesting traffic requires a network connection.