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
Gi1/0/4
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.