How to read Cisco FastEthernet Interface status

Sometimes, by looking over some basic or fundamental information during troubleshooting, we got wrong path and wasted hack of time. If you carefully reviewed interface status, you might easily guess what the root of issue. At least some of clue from there. Understanding interface status is very important step on your troubleshooting path.



Here is simple guide how to read and understand status of FastEthernet interface on Cisco router.
FastEthernet0/0 is up
Indicates the interface hardware is currently active

Line protocol is up
Indicates the software processes that handle the line protocol consider the line usable.

MTU 1500 bytes
Maximum Transmission Unit of the interface.

BW 100000 Kbit
Bandwidth of the interface in kilobits per second. It can be changed by "bandwidth" command

DLY 100 usec
Delay of the interface in microseconds.

reliability 255/255
Reliability of the interface as a fraction of 255 (255/255 is 100% reliability), calculated as an exponential average. If layer 1 or 2 are having an issue, it brings down reliability number. For example; 236/255.

txload 1/255, rxload 1/255
Load on the interface as a fraction of 255 (255/255 is completely saturated). The number is calculated based on bandwidth setting by 'bandwidth' command.

Encapsulation ARPA
Encapsulation method assigned to interface.

Keepalive set (10 sec)
Sending keepalive packet in 10 sec to maintain link.

Full-duplex, 100Mb/s, 100BaseTX/FX
Speed, Data Speed, and Wire Type; Very important indications to troubleshooting

Last clearing of "show interface" counters 00:00:02
self-explanatory; indicated when was last cleared interface. It is important to know when interface took errors.

Queueing strategy: fifo
First In, First Out queuing strategy.

Output queue 0/40, 0 drops; input queue 0/75, 0 drops
Number of packets in output and input queues. If link is heavily congested, output drops will be increased.

30 minute input rate 624000 bits/sec, 254 packets/sec
30 minute output rate 571000 bits/sec, 231 packets/sec
Average number of bits and packets transmitted per second in the last 30 minutes.
Default is 5 minutes. To change 30 seconds interval, use "load-interval" commands at interface level.

runts
While CRC is correct, number of packets that are discarded because they are smaller than the medium's minimum packet size. For instance, any Ethernet packet that is less than 64 bytes is considered a runt.

giants(Jabber)
While CRC is corect, number of packets that are discarded because they exceed the medium's maximum packet size. For example, any Ethernet packet that is greater than 1,518 bytes is considered a giant.

input errors
Total # of errors. It includes runts, giants, no buffer, CRC, frame, overrun, and ignored counts. 

CRC
Cyclic Redundancy Checksum errors are generated by the far-end device does not match the checksum calculated from the data received. It is normally caused by physical layer issues, configuration(duplex) mis-matching and heavy collision on the Ethernet segment. 

frame
Number of packets received incorrectly having a CRC error and abnormal frame structure.

overrun
Number of times the receiver hardware was unable to hand received data to a hardware buffer because the input rate exceeded the receiver's ability to handle the data. If the number is increasing, quickly check cpu utilization by using "show processes cpu". Most of time, CPU utilization would be higher than 80~90%.

ignored
Number of received packets ignored by the interface because the interface hardware ran low on internal buffers. 

watchdog
Number of times watchdog receive timer expired. It happens when receiving a packet with length greater than 2048.

input packets with dribble condition detected
Dribble bit error indicates that a frame is slightly too long. This frame error counter is incremented just for informational purposes; the router accepts the frame.

packets output
Total number of messages transmitted

bytes
Total number of bytes, including data and MAC encapsulation, transmitted by the system.

underruns
Number of times that the transmitter has been running faster than the router can handle. This may never be reported on some interfaces.

output errors
Sum of all errors that prevented the final transmission of datagrams out of the interface being examined. Note that this may not balance with the sum of the enumerated output errors.

collisions
Number of messages retransmitted due to an Ethernet collision. This is usually caused by an overextended LAN, more than two repeaters between stations or too many cascaded multiport transceivers. Some rare cases, mis-matching duplex setting caused collisions as well. If both side hard coded full duplex, collisions counters will remain zero(0).

interface resets
Number of times an interface has been completely reset. This can happen if packets queued for transmission were not sent within several seconds.

babbles
The transmit jabber timer expired.

late collision
Jam signal couldn't reach to ends. It happens, when the network is too big, distance issue between nodes or faulty devices It caused unnecessary traffic on the network.

deferred
Deferred counter implies that the chip had to defer while ready to transmit a frame because the carrier was asserted.

lost carrier
Number of times the carrier was lost during transmission.

no carrier
Number of times the carrier was not present during the transmission.

output buffer failures
Number of no resource errors received on the output.

output buffers swapped out
Number of packets swapped to DRAM.