Topic: Troubleshoot interface and cable issues (collisions, errors, duplex, speed)
Many things can go wrong when you are dealing with a technology as complex as local-area networking, and there are many issues you should be aware of. Note that many of the following issues are not explicitly listed in the CCNA 200-301exam blueprint, but they are very likely to be included on the exam:
▸ The show interface command on a switch displays a ton of potential errors and problems that occur due to interface and cable issues. Notice these errors in the last section of the output shown in Example 3.1.
Example 3.1 show interface Command Output on a Cisco Switch
Switch# show interface gi0/1 GigabitEthernet0/1 is up, line protocol is up (connected) Hardware is iGbE, address is fa16.3eb4.b62b (bia fa16.3eb4.b62b) MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) Unknown, Unknown, link type is auto, media type is unknown media type output flow-control is unsupported, input flow-control is unsupported Auto-duplex, Auto-speed, link type is auto, media type is unknown input flow-control is off, output flow-control is unsupported ARP type: ARPA, ARP Timeout 04:00:00 Last input never, output 00:00:00, output hang never Last clearing of "show interface" counters never Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 32562 Queueing strategy: fifo Output queue: 0/0 (size/max) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 6783 packets input, 0 bytes, 0 no buffer Received 14 broadcasts (0 multicasts) 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 0 watchdog, 0 multicast, 0 pause input 108456 packets output, 7107939 bytes, 0 underruns 0 output errors, 0 collisions, 2 interface resets 0 unknown protocol drops 0 babbles, 0 late collision, 0 deferred 0 lost carrier, 0 no carrier, 0 pause output 0 output buffer failures, 0 output buffers swapped out Switch#
▸ Collisions should not occur in a properly designed switched network. Today, we have the ability to design full-duplex networks using switches that intelligently queue frames to prevent them from being sent simultaneously out an interface.
▸ Errors might occur in a network for a wide variety of reasons. For example, there could be electrical interference somewhere or a bad network interface card that is not able to frame things correctly for the network. Remember that checking the frame check sequence (FCS) is often the best way to catch these errors. Each time a router forwards a packet on an Ethernet network, it replaces and rewrites the Layer 2 Ethernet header information and provides a new FCS.
▸ Duplex used to be a big concern in Ethernet LANs. As described earlier in this chapter, with hubs in a network, in the past you needed to ensure that duplex mismatches did not occur between full-duplex (switched) areas and half-duplex areas. Today, autonegotiation to full-duplex between devices is common. For the CCNA 200-301 exam, you need to understand that if an older device is hard coded to half-duplex, and you code the LAN device connected to full-duplex, a duplex-mismatch may result. Such errors can be difficult to track down because some packets typically make it through the connection fine, whereas others are dropped. In networks that operate in half-duplex, carrier-sense multiple access with collision detection (CSMA/CD) is used to allow devices to operate on a half-duplex network.
▸ Speed is another area where conflict can occur, but this is also becoming a less common problem as technologies advance. For example, 1 Gbps interfaces are quite common now and operate with each other seamlessly at 1 Gbps. The issue, again, is that older equipment might default to a slower speed, causing a speed mismatch.
▸ Runts are Ethernet frames that are less than 64 bytes and may be caused by excessive collisions. Of course, these frames have become rarer as networks have become nearly collision free.
▸ Today many technologies are enhancing networks by adding information to Ethernet frames. This results in Jumbo (or Giant) frames—which are typically frames of 9216 bytes for Gigabit Ethernet but technically can refer to anything over the standard IP MTU (maximum transmission unit) of 1500 bytes.
▸ What if an Ethernet frame is just a little larger than the standard MTU of 1500 bytes? Specifically, what if a frame is 1600 bytes in size? This is what networkers term a Baby Giant frame.