Troubleshooting the physical layer starts with checking for cabling issues. The vast majority of these issues can be overcome if you follow the directions in the documentation of your equipment. Usually, cabling problems are evident at the initial implementation of the service and include issues such as the wrong crossover cable or faulty cables. Later in the chapter, some cable pinpoint examples are discussed. Physical problems are probably the number-one suspect on a circuit that has already been installed and turned up and after time has developed an outage. In most cases, if no one has changed the configurations, it is usually a punchdown, bridge clip, line card, or cable break issue. Line and Clocking ProblemsClocking problems in serial connections can lead to either chronic loss of the connection or to degraded performance. The most common indicators for physical layer problems are shown in Example 17-3. Example 17-3. If This Service Is Provisioned for 56-kbps, Check the Status of Serial 0 1604-frame#show ip interface brief <output omitted> Serial0 unassigned YES NVRAM down down <output omitted> Under 1602-frame#show interface serial 0, the last line reports the status of the physical layer: <output omitted> DCD=down DSR=down DTR=down RTS=down CTS=down <output omitted> These signals are usually referred to as the modem and originally come from RS-232, with the following description:
Although numerous combinations of up and down signals exist, at this stage it is easy to determine what part of the device is failing. DTE is responsible for DTR and RTS, and DCE is responsible for the rest of the signals. When the clockrate command is missing from the DCE side, DCD, DSR, and CTS are shown as inactive or down. If the line is functional but is reporting an excessive number of errors, you should be concerned about the quality of the physical layer. Use the command in Example 17-4 to check the physical layer. Example 17-4. This Part of the Output Is Your Main Focus1604-frame#show interfaces serial 0 <output omitted> 237979 packets input, 21491750 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 5 giants, 0 throttles ! The following line is the part of the output you should review carefully: 198487 input errors, 44907 CRC, 20259 frame, 0 overrun, 0 ignored, 133321 abort 29689 packets output, 3054656 bytes, 0 underruns 0 output errors, 0 collisions, 329 interface res <output omitted> You should also be concerned about a high number of aborts; as you can see in this case, 133321 aborts represent the vast majority of all input errors (about 70 percent). Start with the configuration on both ends. Check the remote user's end first, for interface availability, as shown in Example 17-5. The output in Example 17-5 can be seen from the show version or show hardware commands, which shows that this router has one on-board 56-kbps serial interface. Example 17-5. Output from the show hardware Command 1602-frame#show hardware <output omitted> 1 Serial network interface(s) On-board Switched 56K Line Interface. <output omitted> The clocking issue is addressed in the remote user's router configuration. In the case of a 56-kbps access rate, two configuration commands define the clocking and signaling, as shown in Example 17-6. Example 17-6. A Fragment from the 1602 Configuration, Defining the 56-kbps Access Rate 1602-frame#show running-config <output omitted> ! interface Serial0 no ip address encapsulation frame-relay IETF no fair-queue service-module 56k clock source line service-module 56k network-type dds frame-relay lmi-type ansi ! <output omitted> The service module built into Cisco routers, including the 16xx, 252x, 26xx, 3600, 4xxx, and 7xxx series, supports two modes of 56-kbps settings for network-type: 56-kbps DSU/CSU and 56-kbps switched modes. The second of the following two commands identifies which side provides the clocking: 1602-frame(config-if)#service-module 56k network-type [dds | switched] 1602-frame(config-if)#service-module 56k clock source [line | internal] NOTE Dataphone Digital Service (DDS) provides digital transmission service between the CPE and the provider's switch at data rates between 2.4 kbps and 56 kbps. The service is available over two twisted pair lines.[1] The service module report is shown in Example 17-7. The lines are self-explanatory. Example 17-7. Verifying the Functionality of the Built-In Service Module 1602-frame#show service-module serial 0 Module type is 4-wire Switched 56K in DDS mode, Receiver has no alarms. Current line rate is 56 Kbits/sec and role is DSU side, Last clearing of alarm counters 3d03h oos/oof : 0, loss of signal : 2, last occurred 2d02h loss of sealing current: 0, CSU/DSU loopback : 0, loopback from remote : 0, DTE loopback : 0, line loopback : 0, 1602-frame# In the case of a fractional T1 (or full T1) configuration, the expected output from the configuration should be as displayed in Example 17-8. The output in Example 17-8 can be seen from the show hardware or show version commands and shows that this router has one on-board 56 Kbps interface and one WIC T1-DSU card plugged-in. Example 17-8. Output from the show version Command 1602-frame#show version <output omitted> 2 Serial network interface(s) On-board Switched 56K Line Interface. WIC T1-DSU <output omitted> The configuration settings in the remote user's router are shown in Example 17-9. Example 17-9. This Fragment of the Configuration Defines a 128-kbps Access Rate (2 Slots x 64 kbps Each) 1602-frame#show running-config interface Serial1 <output omitted> encapsulation frame-relay service-module t1 timeslots 1-2 <output omitted> NOTE These settings are important and the remote user needs to be aware of which circuit is ordered and provisioned. A 56-k circuit can be a true 56-k circuit (4 wire or 2 wire), but it can sometimes be a T1 circuit (4 wire, T1 port speed) with 56-k PVC. The WIC-1DSU-T1 cannot be used for 56-k leased line/DDS service, and of course the 56-k WIC cannot be used for T1 service. This configuration defines two slots at 64 kbps each, which results in a 128-kbps access rate. In case the line is provisioned for 128 kbps, 384 kbps, or any other fraction of a T1, the built-in service module (serving a 56-kbps port) slows down because the router does not use this clocking (see Example 17-10). Example 17-10. The Receiver Has Loss of Signal (LOS) because Serial 0 Is Administratively Down. Meanwhile, the WIC Card Is Functioning Properly1602-frame#show service-module Module type is 4-wire Switched 56K in DDS mode, ! You can expect the receiver to lose the signal, because the ! WIC card and DCE will provide the clocking, but not the DDS. Receiver has loss of signal, Current line rate is 56 Kbits/sec and role is DSU side, Last clearing of alarm counters 6w1d oos/oof : 0, loss of signal : 1, current duration 6w1d loss of sealing current: 0, CSU/DSU loopback : 0, loopback from remote : 0, DTE loopback : 0, line loopback : 0, Module type is T1/fractional Hardware revision is 0.88, Software revision is 0.2, Image checksum is 0xED22BEC5, Protocol revision is 0.1 ! This line usually indicates that the WIC card is functioning properly. Receiver has no alarms. Framing is ESF, Line Code is B8ZS, Current clock source is line, Fraction has 2 timeslots (64 Kbits/sec each), Net bandwidth is 128 Kbits/sec. Last module self-test (done at startup): Passed Last clearing of alarm counters 6w1d loss of signal : 0, loss of frame : 0, AIS alarm : 0, Remote alarm : 0, Module access errors : 0, Total Data (last 96 15 minute intervals): 0 Line Code Violations, 0 Path Code Violations 0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 13 Degraded Mins 0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs Data in current interval (781 seconds elapsed): 0 Line Code Violations, 0 Path Code Violations 0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins 0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs 1602-frame# NOTE For troubleshooting FT1 and T1s, see Chapter 6, "Dial Design and Configuration Solutions," or RFC 1232. The clocking and line issues are irrelevant to the situation, when the serial interface reports the output in Example 17-11. Example 17-11. A Typical Output When the Interface Is Administratively Shut Down 1602-frame#show interfaces serial 0 <output omitted> Serial0 is Administratively Down, Line Protocol is Down Hardware is QUICC Serial (with onboard CSU/DSU) <output omitted> The first command is used for an interface, whereas the second command is used for subinterface. In this case, you need to un-shut the interface with either of the following two commands: 1602-frame(config-if)#no shutdown 1602-frame(config-subif)#no shutdown Serial Interface 0 and Line Protocol Is DownWhen the interface serial 0 is not administratively down, the clocking problems have to be your primary focus, especially when the router reports the output in Example 17-12. Example 17-12. The Line Problems Are Evident from This Output 1602-frame#show interfaces serial 0 Serial0 is Down, Line Protocol is Down Hardware is QUICC Serial (with onboard CSU/DSU) <output omitted> In general, this problem occurs because clocking is not available. The following cases describe situations when the serial interface is not administratively down, but the router is still reporting both layers down. This case is typical for new service installations. The root cause of these problems is due to either or both of the following two cases:
One of the most widespread issues is incorrect cabling. In Table 17-1, the correct cabling is specified for the two most common cases: 56-kbps and fractional T1 WIC cards. NOTE Remember that matching pinouts does not mean that the signaling is correct.
NOTE It's always recommended that you refer to the available technical documentation of the customer premises equipment (CPE). The CSU/DSU derives the data clock from the data that passes through it. Therefore, to recover the clock, the CSU/DSU hardware must receive at least one 1-bit value for every 8 bits of data that pass through it; this is known as ones density. Maintaining ones density allows the hardware to recover the data clock reliably. Serial communications use a mechanism called serial clock transmit external (SCTE) terminal timing. SCTE is the clock that is echoed back from the data terminal equipment (DTE) device (for example, a router), to the data communications equipment (DCE) device. When the DCE device uses SCTE instead of its internal clock to sample data from the DTE, it is better able to sample the data without error, even if there is a phase shift in the cable between the CSU/DSU and the router. SCTE is an important element in serial transmissions and it uses access rates that are fractions of a T1. To isolate clocking problems, besides the ones listed earlier, Cisco routers provide a set of tools to troubleshoot the CSU/DSU by using the show, debug, and loopback commands: 1602-frame#debug service-module 1602-frame# show service-module 1602-frame(config-if)# loopback dte 1602-frame(config-if)# loopback line 1602-frame(config-if)# loopback remote Refer to Figure 17-1 for the show service-module command. Note that, in addition to the listed commands, under the T1 controller of the core router, you can use the following useful command: 7206-frame(config-controller)#loopback diag. The first command reports the functionality of the module, as shown in Example 17-13. Example 17-13. Debugging the Service Module 1602-frame#debug service-module Service module debugging is on 1602-frame# 6w5d: SERVICE_MODULE(0): lxt441 interrupt 1 status A7 loop 0 6w5d: SERVICE_MODULE(0): lxt441 interrupt 1 status 87 loop 0 6w5d: SERVICE_MODULE(0): lxt441 interrupt 1 status A7 loop 0 6w5d: SERVICE_MODULE(0): lxt441 interrupt 1 status 87 loop 0 The output reports that the service module is working properly. Case One Problem ResolutionIt is necessary to perform a series of loopback tests, both local and remote, to determine whether the DSU or CSU is causing the problem. Using the following commands, you can loopback the DTE, line, or the remote site: 1602-frame(config-if)# loopback dte 1602-frame(config-if)# loopback line 1602-frame(config-if)# loopback remote It is always good to keep a record of all the different tests performed. In general, when looping and running these tests, always check the number of errors. Different ping tests (samples and sizes) are also recommended during this testing. You must determine whether there is a trend in the accumulation of errors and on what side of the User-Network Interface (UNI) they occur. If errors are accumulating on both ends of the connection (hub site and spoke site), it is most likely because of a DSU problem. If only the DSU end is accumulating errors, there is a problem with that particular end. An excessive number of aborts is a symptom of end-to-end line problems and it is recommended that you work with the local exchange carrier (LEC), who provides the local loop, to isolate the issue. Some Cisco line cards (T1/FT1 WIC) have a loopback button that, when engaged, allows the LEC to loop the spoke side for intrusive testing and clocking. Case Two Problem ResolutionThe problems for this case are related to faulty DSU/CSU, faulty line cards, and incorrect DSU/CSU configurations. An incorrect CSU configuration is usually related to the clocking source. The following questions should be considered:
An incorrect DSU configuration is related to several important checkpoints. The following questions should be considered:
NOTE Always see the Cisco IOS configuration guides and command references for the version of IOS that you are running on your router. You can also refer to available user guides for your CPE. Performance Issues Related to the Physical LayerYou can experience line problems even if the router reports that the line is not down, as shown in the output in Example 17-14. Example 17-14. The Line Shows Up and the Protocol Is Up, But the Performance Issues Exist 1602-frame#show interfaces serial 1 <output omitted> Serial1 is up, line protocol is up <output omitted> However, the connection experiences slow performance, intermittent connectivity, and high latency. Some of the most common reasons for poor performance follow:
A useful tool in this situation is #show interface serial 0, which allows you to not only check the status of the line at any moment (snapshot), but to monitor the dynamics of the changes. This command reports the output in Example 17-15 (see Table 17-2 for an explanation). Example 17-15. Snapshot of the Serial 0 1602-frame#show interfaces serial 0 Serial0 is up, line protocol is up Hardware is QUICC Serial (with onboard CSU/DSU) MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation FRAME-RELAY, loopback not set Keepalive set (10 sec) LMI enq sent 53714, LMI stat recvd 53710, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 0 LMI type is ANSI Annex D frame relay DTE FR SVC disabled, LAPF state down Broadcast queue 0/64, broadcasts sent/dropped 8958/0, interface broadcasts 3 Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters 1w0d Queuing strategy: fifo Output queue 0/40, 0 drops; input queue 1/75, 0 drops 5 minute input rate 0 bits/sec, 2 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 230129 packets input, 44936503 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 1 giants, 0 throttles 7241 input errors, 3594 CRC, 2701 frame, 0 overrun, 0 ignored, 944 abort 229128 packets output, 49424950 bytes, 0 underruns 0 output errors, 0 collisions, 2509 interface resets 0 output buffer failures, 0 output buffers swapped out 6 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up
If at any step of the troubleshooting process, the number of errors exceeds an approximate range of 0.5 percent to 2.0 percent of traffic on the interface, clocking problems are likely to exist somewhere in the WAN. |