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This chapter presents general troubleshooting information and a discussion of tools and techniques for troubleshooting serial connections. The information presented in this document was created from devices in a specific lab environment. The output of the show interfaces serial EXEC command displays information specific to serial interfaces. This section describes how to use the show interfaces serial command to diagnose serial line connectivity problems in a wide area network (WAN) environment. Typically indicates that the router is not sensing a CD signal (that is, CD is not active).
Verify that you are using the proper cable and interface (see your hardware installation documentation). Put the modem, CSU, or DSU in local loopback mode and use the show interfaces serial command to see if the line protocol comes up. If the problem appears to be on the remote end, repeat Step 1 on the remote modem, CSU, or DSU. If the line protocol does not come up in local loopback mode and if the output of the debug serial interface EXEC command shows that the keepalive counter is not incrementing, a router hardware problem is likely. If the line protocol comes up and the keepalive counter increments, the problem is not in the local router.
If the line protocol is still down, there is a possible hardware failure or cabling problem.
Use the show running-config privileged EXEC command to look for any loopback interface configuration command entries. If you find a loopback interface configuration command entry, use the no loopback interface configuration command to remove the loop.
If the CSU or DSU is not configured in manual loopback mode, contact the leased-line or other carrier service for line troubleshooting assistance.
Verify that there are no identical IP addresses using the show running-config privileged EXEC command or the show interfaces EXEC command. If there are duplicate addresses, resolve the conflict by changing one of the IP addresses.
Output drops appear in the output of the show interfaces serial command (see Figure 15-1) when the system is attempting to hand off a packet to a transmit buffer but no buffers are available. Table 15-2 Serial Lines: Increasing Output Drops on Serial Link - This table outlines the possible problem that may cause this symptom and suggests solutions.
Minimize periodic broadcast traffic (such as routing and SAP updates) by using access lists or by other means. Increase the output hold queue size in small increments (for instance, 25 percent), using the hold-queue out interface configuration command. Input drops appear in the output of the show interfaces serial EXEC command (see Figure 15-1) when too many packets from that interface are still being processed in the system. Increase the output queue size on common destination interfaces for the interface that is dropping packets. Reduce the input queue size, using the hold-queue in interface configuration command, to force input drops to become output drops. If input errors appear in the show interfaces serial output (see Figure 15-1), there are several possible sources of those errors. Symptom: An increasing number of input errors in excess of one percent of total interface traffic. Table 15-5: This table describes the various types of input errors displayed by the show interfaces serial command (see Figure 15-1), possible problems that may be causing the errors and the solutions to those problems. Contact your leased-line or other carrier service and have it perform integrity tests on the line. Interface resets that appear in the output of the show interfaces serial EXEC command (see Figure 15-1) are the result of missed keep-alive packets. Table 15-6: This table outlines the possible problems that may cause this symptom and suggests solutions.
When interface resets are occurring, examine other fields of the show interfaces serial command output to determine the source of the problem. Carrier transitions appear in the output of the show interfaces serial EXEC command whenever there is an interruption in the carrier signal (such as an interface reset at the remote end of a link).
Table 15-7 outlines the possible problems that may cause this symptom and suggests solutions.
If an analyzer or breakout box is unable to identify any external problems, check the router hardware.
The show controllers EXEC command is another important diagnostic tool when troubleshooting serial lines. For serial interfaces on Cisco 7000 series routers, use the show controllers cbus EXEC command. On access products such as the Cisco 2000, Cisco 2500, Cisco 3000, and Cisco 4000 series access servers and routers, use the show controllers EXEC command. The show controllers output indicates the state of the interface channels and whether a cable is attached to the interface. The output of the various debug privileged EXEC commands provides diagnostic information relating to protocol status and network activity for many internetworking events. The ping command is a useful test available on Cisco internetworking devices as well as on many host systems. Cisco internetworking devices provide a mechanism to automate the sending of many ping packets in sequence. Examine the show interfaces serial command output (see Figure 15-1) and determine whether input errors have increased. If you determine that the clocking configuration is correct and is operating properly, put the CSU or DSU into remote loopback mode. Clocking conflicts in serial connections can lead either to chronic loss of connection service or to degraded performance. Newer T1 implementations commonly use Extended Superframe Format (ESF) framing with binary eight-zero substitution (B8ZS) coding.
Older T1 implementations use D4-also known as Superframe Format (SF) framing and Alternate Mark Inversion (AMI) coding.
Another important element in serial communications is serial clock transmit external (SCTE) terminal timing. Determine the end of the connection that is the source of the problem, or if the problem is in the line. Use the show interfaces serial EXEC command and determine if input errors counts are increasing and where they are accumulating. If input errors are accumulating on both ends of the connection, clocking of the CSU is the most likely problem. If only one end is experiencing input errors, there is probably a DSU clocking or cabling problem. Aborts on one end suggests that the other end is sending bad information or that there is a line problem. Check the LBO setting on the CSU to ensure that the impedance matches that of the physical line. If your carrier service uses AMI coding, either invert the transmit clock on both sides of the link or run the DSU in bit-stuff mode. To ensure that you are using the correct command syntax for your router, refer to the user guide for your router or access server and to the Cisco IOS configuration guides and command references.
Excessively high bandwidth utilization (over 70percent) results in reduced overall performance and can cause intermittent failures.
The configuration commands associated with these options are described in the Cisco IOS configuration guides and command references. The system buffers are associated with the main system memory and are allocated to different-size memory blocks. The min parameter is used to anticipate demand for buffers from the pool at any given time.
If the number of buffers in the free list falls below the min value, the RP attempts to create more buffers for that pool. The max allowed parameter prevents a pool from monopolizing buffers that it doesn't need anymore and frees this memory back to the system for further use. If the number of buffers in the free list is greater than the max allowed value, the RP should attempt to trim buffers from the pool. The show buffers command output in Figure 15-8 indicates high numbers in the trims and created fields for large buffers.


Use the buffers max free number global configuration command to increase the number of free system buffers.
If the show buffers command output shows a large number of failures in the (no memory) field (see the last line of output in Figure 15-8), you must reduce the usage of the system buffers or increase the amount of shared or main memory (physical RAM) on the router.
Hold queues are buffers used by each router interface to store outgoing or incoming packets. You have an application that cannot tolerate drops and the protocol is able to tolerate longer delays. Priority queuing is a list-based control mechanism that allows traffic to be prioritized on an interface-by-interface basis. When the interface is slow, there is a variety of traffic types being transmitted, and you want to improve terminal traffic performance.
If you have a serial link that is intermittently experiencing very heavy loads (such as file transfers occurring at specific times) priority queuing will help select which types of traffic should be discarded at high traffic periods.
In addition to the basic diagnostic capabilities available on routers, a variety of supplemental tools and techniques can be used to determine the conditions of cables, switching equipment, modems, hosts, and remote internetworking hardware. If the line protocol comes up when the CSU or DSU is in local loopback mode, this suggests that the problem is occurring on the remote end of the serial connection. If the problem appears to be local, use the debug serial interface privileged EXEC command. Figure 15-10 shows the output from the debug serial interface command for an HDLC serial connection, with missed keepalives causing the line to go down and the interface to reset. If you determine that the local hardware is functioning properly but you still encounter problems when attempting to establish connections over the serial link, try using the remote loopback test to isolate the problem cause. The following sub-sections cover the show interfaces serial command's parameters, syntax description, sample output display, and field descriptions.
T1 channels on the CT3IP are numbered 1 to 28 rather than the more traditional zero-based scheme (0 to 27) used with other Cisco products. Table 15-9: show interfaces serial Field Descriptions - this table describes significant fields shown in the output. Indicates whether the interface hardware is currently active (carrier detect is present) or whether it has been taken down by an administrator. Indicates whether the software processes that handle the line protocol consider the line usable (that is, keepalives are successful) or whether it has been taken down by an administrator. Indicates the value of the bandwidth parameter that has been configured for the interface (in kilobits per second). Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts.
I'm new to wiring and I have this RJ11 to DB9 RS232 adapter, and I'm not sure how to correctly insert the RJ11 wires into the DB9 female end.
The reason that they did not connect the RJ11 connector to the DB9 is that there IS no standard way available. The reason that they left the wires uncommitted was so that you could connect them in the way that suits you. Worst case you may have some equipment that has a serial circuit on an RJ11 connector that you want to connect to.
This is just an overview -m knowing the equipment brand and model or, failing that, type of equipment and intended use will help geatly.
Examples - these are useful mainly to show how thoroughly non standard the various arrangements are.
Here's a plug pinout from one of their several arrangements - it's notable for being different from but similar to the Dell arrangement above.
The db9 connector will have the pin position numbers stamped on the front of the connector. Not the answer you're looking for?Browse other questions tagged serial db9 or ask your own question. How do I tell a serial port settings, when the hardware documentation doesn't have any info about it? Is it acceptable for a student to ask a professor to be included as a co-author on the professor's paper? Important disclaimer: While unlikely, it could still be possible to damage your router completely by following the steps described in this page, we take no responsibility for any damage which could be possibly caused. Our network lab consists our of 13 different devices, mainly Cisco and is used by us to simulate different network environments for our projects. The malfunctions in 5 devices manifested in ability to see the output, but not type in and one console died completely. We still do not exactly know what caused this problem, we used this setup for quite a while without any problems, but we are pretty sure that our "plug-unplug console cable" game made this thing possible in the first place.
First, our searches on the net resulted in nothing, we have seen that many people had this problem before, but all of the answers where basically "RMA or buy new" advises. Well, our machines are not under support contract, so we cannot RMA for free and repair by Cisco would have cost us more than the whole new machines off eBay. First thing you need is are the replacement chip(s), you can ether buy them if you find a source, or you can find a donor router and get the chips from there, which was our choice. First of all you need to know which chip to remove, you can find the correct one by ether following mainboard leads from the console port resistors or just taking the chip nearest to the console port. Again, you can ether use advanced methods of removing the fluid tin by sucking out or absorbing it, or you can simply bend the chips's legs when they heat up well enough. Another way to loosen the chip is to put a lot of tin across all the legs of the chip, a "tinbridge", first one side and then the other of the chip then quickly back to the first side and genltly push the chip sideways with the soldering iron tip. First of all, make sure you seat the new chip in correct direction, same way the previous was seated. It would not hurt to add some very small ammounts of tin to the leads on mainboard prior of seating the chip in place, just to make sure the contacts are really good.
Use tweezers to position the chip, then, while still holding the chip in its place with tweezers push gently each leg down with soldering iron, the tin on the mainboard and chip legs should connect flawlessly. Figure 15-1 shows the output of the show interfaces serial EXEC command for a High-Level Data Link Control (HDLC) serial interface. If the line protocol comes up, a telephone company problem or a failed remote router is the likely problem. The sequence number in the keepalive packet changes to a random number when a loop is initially detected. For example, to increase the delay between SAP updates, use the ipx sap-interval interface configuration command. For example, to turn off IP fast switching, enter the no ip route-cache interface configuration command. For information on configuring priority lists, see the Cisco IOS configuration guides and command references.
For instance, if a link is known to be overused (with no way to remedy the situation), it is often preferable to drop packets than to hold them. If you detect errors, it is likely that there is a hardware problem or a clock mismatch in a device that is external to the router. For example, if errors occur at a consistent interval, they could be related to a periodic function such as the sending of routing updates. If carrier transitions are high while interface resets are being registered, the problem is likely to be a bad link or bad CSU or DSU. This command is used on Cisco 7000 series routers with the Fast Serial Interface Processor (FSIP) card. Figure 15-3 shows the show controllers command output from the Basic Rate Interface (BRI) and serial interfaces on a Cisco 2503 access server.
For this reason, use debug commands only to troubleshoot specific problems or during troubleshooting sessions with Cisco technical support staff. This display also prints error messages to indicate why a packet was not sent or was received erroneously. Figure 15-6 and Figure 15-7 illustrate two useful ping tests, an all-zeros (1500-byte) ping and an all-ones (1500-byte) ping, respectively.
If input errors have not increased, the local hardware (DSU, cable, router interface card) is probably in good condition. This section discusses the important aspects of clocking problems: clocking problem causes, detecting clocking problems, isolating clocking problems, and clocking problem solutions. B8ZS provides a scheme by which a special code is substituted whenever eight consecutive zeros are sent through the serial link. In local loopback mode, run different patterns and sizes in the ping tests (for example, use 1500-byte datagrams).


This requires that the DSU use the same framing and coding schemes (for example, ESF and B8ZS) used by the leased-line or other carrier service. Inverting the transmit clock compensates for phase shifts between the data and clock signals. On a Cisco 7000 series router, enter the invert-transmit-clock interface configuration command. For example, DECnet file transmissions may be failing due to packets being dropped somewhere in the network. You can severely affect the performance of your hardware and your network if you incorrectly adjust the system buffers on your router. A useful command for determining the status of your system buffers is the show buffers EXEC command.
The hits counter provides a mechanism for determining which pool must meet the highest demand for buffers. The number of failures represents the number of packets that have been dropped due to buffer shortage.
If you are receiving high numbers in these fields, you can increase your serial link performance by increasing the max free value configured for your system buffers. The value you configure should be approximately 150 percent of the figure indicated in the total field of the show buffers command output.
Use the hold-queue interface configuration command to increase the number of data packets queued before the router will drop packets.
Bandwidth is low or anticipated utilization is likely to sporadically exceed available bandwidth. The value used depends on the size of the packets associated with the traffic anticipated for the network.
In addition, further traffic control can be applied by referencing access-list global configuration commands from priority-list specifications.
After enabling priority queuing, monitor output drops with the show interfaces serial EXEC command. A high-priority queue depth of about 100 (specified with the queue-limit keyword) is a typical working value when your router is dropping output packets and the serial lines are subjected to about 50 percent bandwidth utilization. You can implement LAT compression with the interface configuration command bridge-group group lat-compression. For more information, consult the documentation for your CSU, DSU, serial analyzer, or other equipment. In local loop mode, the use of the line clock (from the T1 service) is terminated, and the DSU is forced to use the local clock. When the line protocol is down, the debug serial interface command output will indicate that keepalive counters are not incrementing. Displays the number of packets of each protocol type that have been sent through the interface. On the Cisco 4000 series with an NPM or a Cisco 7500 series with a MIP, specifies the T1 channel-group number in the range of 0 to 23, defined with the channel-group controller configuration command. This is to ensure consistency with Telco numbering schemes for T1 channels within channelized T3 equipment. It's hard to see which cable is which, but there are six: black, white, green, red, blue, and yellow. That way, you could run serial over existing wiring, but plugging a ethernet connection into the serial port would take some effort.
You may have a good reason but (as noted below) RJ11 is non standard for this purpose and you are inviting compatability problems.
If you feel insecure about soldering on electronic equipment, ask someone who is skilled in that to do it for you. Until now, when we needed the console on some devices, we just plugged the notebook or workstation's serial cable into that device. Lessons learned: Use console server as much as possible, avoid plugging and unplugging serial console cables while equipment is on.
Whatever you do, make sure that you do not damage the mainboard, do not use force to remove the chip or you might end up with damage on mainboard's leads, if you still do damage the board, do not panic, you can fix it by tracing where those leads went and replace them by thin wires (See images in next section). If you are working in a live network, ensure that you understand the potential impact of any command before using it. Use the show controllers EXEC command to determine which cable is attached to which interface.
For the command syntax for other protocols, consult the Cisco IOS configuration guides and command references.
Increase these queues by small increments (for instance, 25percent) until you no longer see drops in the show interfaces output. Moreover, it is best to use debug commands during periods of low network traffic and fewer users.
For SMDS, the command dumps the entire SMDS header and some payload data when an SMDS packet is transmitted or received. Assuming that this test sequence was prompted by the appearance of a large number of CRC and framing errors, a clocking problem is likely. This restricts the type of data that can be transmitted because ones density is not maintained independent of the data stream.
One way to resolve marginal serial line overutilization problems is to control how the router uses data buffers. The hardware buffers are specifically used as the receive and transmit buffers associated with each interface and (in the absence of any special configuration) are dynamically managed by the system software itself. Repeat this process until the show buffers output no longer indicates trims and created buffers.
Increase these queues by small increments (for instance, 25 percent) until you no longer see drops in the show interfaces output. For examples of defining priority lists and for details about command syntax associated with priority queuing, refer to the Cisco IOS configuration guides and command references. If you notice that output drops are occurring in the traffic queue you have specified to be high priority, increase the number of packets that can be queued (using the queue-limit keyword option of the priority-list global configuration command).
If the router is dropping packets and is at 100 percent utilization, you need another line.
Specifically, the values for mineseen and yourseen keepalives will increment every 10 seconds. If the interface is attached to a serial line with a line speed that does not match the default (1536 or 1544 for T1 and 56 for a standard synchronous serial line), use the bandwidth command to specify the correct line speed for this serial line.
You should also certany not perform this steps if you have a support contract with cisco since this will void your warranty.
So we decided to try and repair the broken machines, which turned out to be really easy and we where quite surprised that no one mentioned this option yet.
After you finish with all legs you should try to remove chip, if it is still stuck try to find out which legs are still soldered on and repeat the "heat up, bend" procedure.
However, some protocols, such as DECnet and local-area transport are sensitive to dropped packets and accommodate retransmission poorly, if at all. Debugging during these periods decreases the likelihood that increased debug command processing overhead will affect system use. However, when testing the components on your serial line, you should specify a much larger number, such as 1000 pings. The default queue-limit arguments are 20 packets for the high-priority queue, 40 for medium, 60 for normal, and 80 for low. Care should be taken to insert the correct pins as they can not be removed without a removal tool. And last but not the least: you could sacrifice some of the router functionality like aux or serial ports and use their chips to repair console. If the electrical interface is unknown, the corresponding display for the show interfaces serial EXEC command will show that the interface and line protocol are down. When you finish using a debug command, remember to disable it with its specific no debug command or with the no debug all command.
If the keepalives do not increment, there may be a timing problem on the interface card or on the network. As far as the pin out of the RJ11 you need to see what device it connects to and which pins correspond to the db9.



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