Troubleshooting with the show CommandsThe show commands are used to troubleshoot the Physical layer of the network. The following sections discuss each of the commonly used show commands in more detail. These sections also display and explain the show command's output.
The show interfaces CommandOne of the most common show commands that you will use is show interfaces , which enables you to see what is happening on all the interfaces on the router. It gives you a quick glance at an interface; the show interfaces command indicates whether the network is up or down, provides a bit of information on network configuration, and provides a look at the packets on the network. The following is example output from show interfaces : DCSRootRTR# show interfaces Async1 is down, line protocol is down Hardware is Async Serial MTU 1500 bytes, BW 9 Kbit, DLY 100000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation SLIP, loopback not set Ethernet2/1 is up, line protocol is up Hardware is cxBus Ethernet, address is 1203.ef12.1d41 (bia 1203.ef12.1d41) Description: Subnet 211 for Pismo Beach Site Internet address is 192.16.32.254/24 MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, reliability 255/255, txload 4/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) ARP type: ARPA, ARP Timeout 04:00:00 Last input 00:00:00, output 00:00:05, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 22591 drops; input queue 0/75, 929 drops 5 minute input rate 44000 bits/sec, 25 packets/sec 5 minute output rate 174000 bits/sec, 23 packets/sec 293110943 packets input, 4194038437 bytes, 0 no buffer Received 42837453 broadcasts, 0 runts, 0 giants, 71 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 25 ignored 0 input packets with dribble condition detected 264542130 packets output, 1810892381 bytes, 0 underruns 0 output errors, 31586499 collisions, 2 interface resets 0 babbles, 0 late collision, 0 deferred 0 lost carrier, 0 no carrier 0 output buffer failures, 0 output buffers swapped out Ethernet2/2 is administratively down, line protocol is down Hardware is cxBus Ethernet, address is 1203.ef12.1d42 (bia 1203.ef12.1d42) Description: Subnet 212 Monterey Site Internet address is 198.132.212.254/24 MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) ARP type: ARPA, ARP Timeout 04:00:00 Last input never, output never, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec DCSRootRTR# The output above shows a serial interface on a Cisco 3725 router. By looking at the second line, you get a picture of an unhealthy interface. The interface will not connect and the Layer 3 protocols running on the interface are not receiving keepalives . Keepalive packets are sent by Layer 3 protocols at specific intervals and whenever there is a topology change in the network to verify that the interface is still active and functioning correctly. You know this because both the line and protocol identifiers are showing they are down. If you look at the line in bold you will also note that there was a very large number of collisions, which could signal either a hardware problem or a problem with the link itself.
The show interfaces ethernet CommandUnfortunately, you can't just type in the show interfaces ethernet command and see all the Ethernet interfaces on the router. You have to identify the interface you want to view after the command. This command allows you to examine the status of an Ethernet and get in-depth information on that interface's problems (if there are any). The following example shows the first Ethernet interface on the router indicated by the zero at the end of the show interfaces ethernet command: DCSRoot# show interfaces ethernet 0 Ethernet0 is up, line protocol is up Hardware is PQUICC Ethernet,address is 0007.eb32.d6a3 (bia 0007.eb32.d6a3) Internet address is 207.212.78.110/29 MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) Half-duplex, 10BaseT ARP type: ARPA, ARP Timeout 04:00:00 Last input 00:00:00, output 00:00:00, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 329 drops 5 minute input rate 44000 bits/sec, 7 packets/sec 5 minute output rate 4000 bits/sec, 7 packets/sec 8812047 packets input, 539578735 bytes, 0 no buffer Received 15611 broadcasts, 0 runts, 0 giants, 0 throttles 475 input errors, 0 CRC, 1 frame, 0 overrun, 474 ignored 0 input packets with dribble condition detected 7040396 packets output, 1116153898 bytes, 0 underruns 55169 output errors, 71884 collisions, 8 interface resets 0 babbles, 0 late collision, 0 deferred 474 lost carrier, 0 no carrier Let's go line by line and break down the preceding output to see what each line is telling us:
The show interfaces tokenring CommandThe show interfaces tokenring command is used to show the status of a Token Ring interface. Just like in the show interfaces ethernet command, this command must be followed by the interface number as shown below: DCSRouter2# show interface tokenring 0 TokenRing0 is up, line protocol is down Hardware is TMS380, address is 0002.cda1.26ac (bia 0002.cda1.26ac) Internet address is 172.16.21.254, subnet mask is 255.255.255.0 MTU 4464 bytes, BW 4000 Kbit, DLY 630 usec, rely 255/255, load 1/255 Encapsulation SNAP, loopback not set, keepalive set (10 sec) ARP type: SNAP, ARP Timeout 04:00:00 Ring speed: 4 Mbps Single ring node, Source Route Transparent Bridge capable Ethernet Transit OUI: 0x000000 Last input never, output never, output hang never Last clearing of "show interface" counters never Queueing strategy: fifo Output queue 0/40, 0 drops; input queue 0/75, 0 drops 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 0 packets input, 0 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 0 packets output, 0 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 output buffer failures, 0 output buffers swapped out 1 transition The preceding output is very similar to the Ethernet output and has many of the same fields. Let's take a look at only the fields that are different from the show interfaces ethernet command:
The show controllers CommandThe show controllers command output shows information on all of the router's controllers. You can specify the type of controller when using this command, as shown below: DCSSacramento# show controllers ? Ethernet IEEE 802.3 FastEthernet FastEthernet IEEE 802.3 Output modifiers <cr> Although the preceding output only lists Ethernet and FastEthernet interfaces, routers with other types of interfaces, such as serial, BRI, CBus, FDDI, Lex, MCI, PCBus, serial, T1, T3, Token, or VGAnylan controllers, can also be specified. This is the command you want to use if you want to view the versions of firmware being used, types of memory management, or another view of the errors detected on interfaces on the router. Much of the information displayed here is unusable, but in between is information that can be used to troubleshoot Physical layer interface problems. Let's take a look at the output from the Ethernet 0 interface on a Cisco 1720 router shown below: RTR# show controllers ethernet0 PQUICC Ethernet unit 0 using SCC1, Microcode ver 0 Current station address 0007.eb32.d6a3, default address 8167.0210.8107 idb at 0x81310358, driver data structure at 0x81312140 SCC Registers: General [GSMR]=0x0:0x1088003C, Protocol-specific [PSMR]=0x80A Events [SCCE]=0x0000, Mask [SCCM]=0x001F, Status [SCCS]=0x0002 Transmit on Demand [TODR]=0x0, Data Sync [DSR]=0xD555 Interrupt Registers: Config [CICR]=0x00365F80, Pending [CIPR]=0x0400C000 Mask [CIMR]=0x40200000, In-srv [CISR]=0x00000000 Command register [CR]=0x600 Port A [PADIR]=0x0000, [PAPAR]=0x0903 [PAODR]=0x0000, [PADAT]=0xF7FF Port B [PBDIR]=0x0000100F, [PBPAR]=0x0000100E [PBODR]=0x00000000, [PBDAT]=0x0003EDFC Port C [PCDIR]=0x0000, [PCPAR]=0x0000 [PCSO]=0x0030, [PCDAT]=0x0FCC, [PCINT]=0x0000 IPM_SYS_REGS->tecr is0x11 SCC GENERAL PARAMETER RAM (at 0xFF003C00) Rx BD Base [RBASE]=0x2230, Fn Code [RFCR]=0x18 Tx BD Base [TBASE]=0x22B0, Fn Code [TFCR]=0x18 Max Rx Buff Len [MRBLR]=1520 Rx State [RSTATE]=0x18000000, BD Ptr [RBPTR]=0x2258 Tx State [TSTATE]=0x18000AE3, BD Ptr [TBPTR]=0x22C0 SCC ETHERNET PARAMETER RAM (at 0xFF003C30) CRC Preset [C_PRES]=0xFFFFFFFF, Mask [C_MASK]=0xDEBB20E3 Errors: CRC [CRCEC]=0, Alignment [ALEC]=1, Discards [DISFC]=0 PAD Char [PADS]=0x0 Retry Limit [RET_LIM]=15, Count [RET_CNT]=15 Frame Lengths: [MAXFLR]=1518, [MINFLR]=64 Max DMA Lengths: [MAXD1]=1518, [MAXD2]=1518 Group Address Filter [GADDRn]=0000:0000:0020:0100 Indiv Address Filter [IADDRn]=0000:0000:0000:0000 Physical Address [PADDR1]=A3D6.32EB.0700 Last Address Set in Filter [TADDR]=CCCC.CC0C.0001 Persistence [P_Per]=0, Backoff Cnt [BOFF_CNT]=65535 BD Pointers: First Rx [RFBD]=0x0, First Tx [TFBD]=0x22C0, Last Tx [TLBD]=0x22B8 Receive Ring rmd(FF002230): status 9000 length 4D4 address 1CA0CA4 rmd(FF002238): status 9000 length 262 address 1CA26A4 rmd(FF002240): status 9000 length 3B6 address 1C9C524 rmd(FF002248): status 9000 length 5EE address 1CA3A24 rmd(FF002250): status 9000 length 5EE address 1C9DF24 rmd(FF002258): status 9000 length 4EC address 1C9FFA4 rmd(FF002260): status 9000 length 8A address 1C9AB24 rmd(FF002268): status 9000 length 5EE address 1C9FFA4 rmd(FF002270): status 9000 length 5EE address 1C9E5A4 rmd(FF002278): status 9000 length 5EE address 1C97724 rmd(FF002280): status 9000 length 5EE address 1CA40A4 rmd(FF002288): status 9000 length 1F3 address 1CA2024 rmd(FF002290): status 9000 length 5EE address 1C9A4A4 rmd(FF002298): status 9000 length 5EE address 1CA2D24 rmd(FF0022A0): status 9000 length 48A address 1C9F924 rmd(FF0022A8): status B000 length 5EE address 1C99E24 Transmit Ring tmd(FF0022B0): status 5C00 length 3C address 1B343CA tmd(FF0022B8): status 5C00 length 3C address 1B343CA tmd(FF0022C0): status 5C00 length 3C address 1B343CA tmd(FF0022C8): status 5C00 length 3C address 1B343CA tmd(FF0022D0): status 5C00 length 159 address 1B35DCA tmd(FF0022D8): status 5E00 length 3C address 1B343CA tmd(FF0022E0): status 5C00 length 3C address 1B343CA tmd(FF0022E8): status 5C00 length 3C address 1B343CA tmd(FF0022F0): status 5C00 length 3C address 1B343CA tmd(FF0022F8): status 5C00 length 3C address 1B343CA tmd(FF002300): status 5C00 length 15D address 1B35DCA tmd(FF002308): status 5C00 length 15C address 1B35DCA tmd(FF002310): status 5E00 length 3C address 1B343CA tmd(FF002318): status 5C00 length 151 address 1B3748A tmd(FF002320): status 5C00 length 3C address 1CD9C8A tmd(FF002328): status 7C00 length 3C address 1CD9C8A 483 missed datagrams, 0 overruns 0 transmitter underruns, 0 excessive collisions 44446 single collisions, 35471 multiple collisions 0 dma memory errors, 0 CRC errors 1 alignment errors, 0 runts, 0 giants QUICC SCC specific errors: 483 buffer errors, 0 overflow errors 0 input aborts on late collisions 0 throttles, 0 enables Most of the output above is only useful to the Cisco TAC that can decifer it. However, the bolded lines at the end show a tremendous amount of useful information about the errors received on the interface. The same information, however, can be learned by other troubleshooting commands, such as the show interfaces command. The show ip traffic CommandThe show ip traffic command is used to display traffic statistics showing the numbers of IP packets that were sent and received for each IP protocol. It also shows the numbers of errors detected, numbers of multicasts, and the number of broadcasts sent and received by the interface. This command is useful because it reveals protocol problems on the interface that are not shown by the show ip interfaces command. An example of the command is shown below: DCSRTR#show ip traffic IP statistics: Rcvd: 169054310 total, 1038115 local destination 0 format errors, 0 checksum errors, 21202 bad hop count 559 unknown protocol, 1 not a gateway 0 security failures, 0 bad options, 789 with options Opts: 0 end, 0 nop, 0 basic security, 0 loose source route 230 timestamp, 0 extended security, 0 record route 0 stream ID, 0 strict source route, 559 alert, 0 cipso, 0 ump 0 other Frags: 5 reassembled, 13 timeouts, 0 couldn't reassemble 1336 fragmented, 0 couldn't fragment Bcast: 953104 received, 305351 sent Mcast: 1147 received, 0 sent Sent: 581960 generated, 167783917 forwarded Drop: 185754 encapsulation failed, 0 unresolved, 0 no adjacency 20594 no route, 0 unicast RPF, 0 forced drop ICMP statistics: Rcvd: 0 format errors, 1 checksum errors, 14 redirects, 19615 unreachable 1036 echo, 40 echo reply, 0 mask requests, 0 mask replies, 2 quench 0 parameter, 0 timestamp, 0 info request, 0 other 0 irdp solicitations, 5882 irdp advertisements Sent: 162077 redirects, 36604 unreachable, 850 echo, 1036 echo reply 0 mask requests, 0 mask replies, 0 quench, 0 timestamp 0 info reply, 21214 time exceeded, 0 parameter problem 0 irdp solicitations, 0 irdp advertisements TCP statistics: Rcvd: 37767 total, 2 checksum errors, 7018 no port Sent: 25963 total UDP statistics: Rcvd: 973075 total, 0 checksum errors, 972367 no port Sent: 335235 total, 0 forwarded broadcasts BGP statistics: Rcvd: 0 total, 0 opens, 0 notifications, 0 updates 0 keepalives, 0 route-refresh, 0 unrecognized Sent: 0 total, 0 opens, 0 notifications, 0 updates 0 keepalives, 0 route-refresh OSPF statistics: Rcvd: 0 total, 0 checksum errors 0 hello, 0 database desc, 0 link state req 0 link state updates, 0 link state acks Sent: 0 total IP-EIGRP statistics: Rcvd: 0 total Sent: 0 total PIMv2 statistics: Sent/Received Total: 0/0, 0 checksum errors, 0 format errors Registers: 0/0, Register Stops: 0/0, Hellos: 0/0 Join/Prunes: 0/0, Asserts: 0/0, grafts: 0/0 Bootstraps: 0/0, Candidate_RP_Advertisements: 0/0 State-Refresh: 0/0 IGMP statistics: Sent/Received Total: 0/0, Format errors: 0/0, Checksum errors: 0/0 Host Queries: 0/0, Host Reports: 0/0, Host Leaves: 0/0 DVMRP: 0/0, PIM: 0/0 ARP statistics: Rcvd: 1177966 requests, 1050 replies, 110 reverse, 0 other Sent: 181941 requests, 96748 replies (441 proxy), 0 reverse DCSRTR# The above output is helpful because it separates the network statistics, including errors by protocol. This enables you to rule out certain protocols as being problems during troubleshooting.
show arpAnother show command that may be useful is the show arp command, which is very helpful when you're trying to locate a particular device on your network. The ARP cache records all the devices that have traversed the router. The ARP cache lasts only for a short time after the device is no longer on the network, so this command is time-sensitive. By using this command, you can find the MAC address and associate it to an IP address on the network, enabling you to see how long the device was on the network. The show arp command also identifies which interface the devices are using. The following is example output from show arp : DCSRootRTR# show arp Protocol Address Age (min) Hardware Addr Type Interface Internet 192.16.33.254 0083.12eb.1dd5 ARPA Ethernet2/1 Internet 192.16.34.224 3 00a0.b8be.9512 ARPA Ethernet2/4 Internet 192.16.35.65 63 0080.5abc.55b4 ARPA Ethernet2/5 The above output shows the ARP table from the router, which shows the MAC address to IP resolutions the router has made, as well as the interface to reach each host. |