Basic troubleshooting

Show interfaces

>show interfaces status

Port      Name               Status       Vlan       Duplex  Speed Type
Fa1/0/1                      notconnect   1            auto   auto 10/100BaseTX
Fa1/0/2                      notconnect   1            auto   auto 10/100BaseTX
Fa1/0/3                      notconnect   1            auto   auto 10/100BaseTX
Fa1/0/4                      notconnect   1            auto   auto 10/100BaseTX
Fa1/0/47                     notconnect   1            auto   auto 10/100BaseTX
Fa1/0/48                     connected    1          a-full  a-100 10/100BaseTX
Gi1/0/1                      notconnect   1            auto   auto Not Present
Gi1/0/2                      notconnect   1            auto   auto Not Present
Gi1/0/3                      notconnect   1            auto   auto Not Present
Gi1/0/4                      notconnect   1            auto   auto Not Present
>show interfaces fa1/0/48
FastEthernet1/0/48 is up, line protocol is up (connected)
    Hardware is Fast Ethernet, address is 0015.2b2e.2634 (bia 0015.2b2e.2634)
    MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
        reliability 255/255, txload 1/255, rxload 1/255
    Encapsulation ARPA, loopback not set
    Keepalive set (10 sec)
    Full-duplex, 100Mb/s, media type is 10/100BaseTX
    input flow-control is off, output flow-control is unsupported
    ARP type: ARPA, ARP Timeout 04:00:00
    Last input 00:00:12, 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: 0
    Queueing strategy: fifo
    Output queue: 0/40 (size/max)
    5 minute input rate 0 bits/sec, 0 packets/sec
    5 minute output rate 0 bits/sec, 0 packets/sec
        72 packets input, 6636 bytes, 0 no buffer
        Received 52 broadcasts (0 multicast)
        0 runts, 0 giants, 0 throttles
        0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
        0 watchdog, 18 multicast, 0 pause input
        0 input packets with dribble condition detected
        155 packets output, 23576 bytes, 0 underruns
        0 output errors, 0 collisions, 1 interface resets
        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

The first line indicates the layer1 and layer2 status of the interface.

Layer 1 Layer 2 Status
up up Ok
up down Connection issues, not receiving the L2 keepalive
down down (notconnect) Cable is not connected to at least one switch, or other switch port disabled.
down down Layer 1 issue or other switch port administratively shut down.
adinistratively down down Interface administratively shut down

The last block gives statistics on the packets.

Parameter Description
runts Frame too small and bad CRC (<46 bytes for ethernet)
giants Frame too big and bad CRC (>1518 bytes for non-jumbo ethernet)
input errors Sum of input errors (runts, giants, CRC etc)
output errors Sum of packets that haven’t been transmitted because of errors
collisions Number of collisions that occurred (in half duplex mode)
late collisions Number of collisions that happened after the slot time
CRC Incorrect CRC. CRC is a type of FCS

Common errors

Duplex mismatch

It’s easy to get a duplex mismatch, if one swith is trying to autonegociate the duplex mode and the other switch is fixed either to half or full duplex.

One of the tell-tale symptoms is having lots of CRC errors on the full-duplex side, and lots of late collisions on the half-duplex side.

Clear counters

After solving (or trying to solve) an issue, clear the error counters.

#clear counters
Clear "show interface" counters on all interfaces [confirm]
00:31:40: %CLEAR-5-COUNTERS: Clear counter on all interfaces by console

VLAN troubleshooting

>show vlan brief

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa1/0/1, Fa1/0/2, Fa1/0/3
                                                Fa1/0/4, Fa1/0/5, Fa1/0/6
                                                Fa1/0/7, Fa1/0/8, Fa1/0/9
                                                Fa1/0/10, Fa1/0/11, Fa1/0/12
                                                Fa1/0/13, Fa1/0/14, Fa1/0/15
                                                Fa1/0/16, Fa1/0/17, Fa1/0/18
                                                Fa1/0/19, Fa1/0/20, Fa1/0/21
                                                Fa1/0/22, Fa1/0/23, Fa1/0/24
                                                Fa1/0/25, Fa1/0/26, Fa1/0/27
                                                Fa1/0/28, Fa1/0/29, Fa1/0/30
                                                Fa1/0/31, Fa1/0/32, Fa1/0/33
                                                Fa1/0/34, Fa1/0/35, Fa1/0/36
                                                Fa1/0/37, Fa1/0/38, Fa1/0/39
                                                Fa1/0/40, Fa1/0/41, Fa1/0/42
                                                Fa1/0/43, Fa1/0/44, Fa1/0/45
                                                Fa1/0/46, Fa1/0/47, Fa1/0/48
                                                Gi1/0/1, Gi1/0/2, Gi1/0/3
100  ADMIN                            active
1002 fddi-default                     act/unsup
1003 token-ring-default               act/unsup
1004 fddinet-default                  act/unsup
1005 trnet-default                    act/unsup

In the example above, the VLAN 100 exists, but no port is assigned. That could be the sign of an error if some machines can’t communicate together.

If you delete a VLAN that has ports assigned to it, the ports are no longer assigned to any VLAN and they’re not going to be able to communicate.

Before deleting a VLAN, make sure you remove any port assigned to this VLAN.