Thursday, December 21, 2017

What causes the overrun counter to increment on a Cisco switch?

I'm looking for a plain English explanation of the overrun counter.

We have a two member port-channel where this counter is incrementing. According to this page, overruns happen when "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." At first glance it seems like this would mean that the server is sending more traffic than the port can handle. Is this right? The server NICs are 1 gig and the switchports are 1 gig, how could they possibly be overloaded? Also, this port doesn't seem to have been getting much traffic.

Port-channel56 is up, line protocol is up (connected) Hardware is EtherChannel, address is MTU 1500 bytes, BW 2000000 Kbit/sec, DLY 10 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) Full-duplex, 1000Mb/s, media type is N/A input flow-control is off, output flow-control is unsupported Members in this channel: Gi1/8 Gi1/10 ARP type: ARPA, ARP Timeout 04:00:00 Last input never, output never, output hang never Last clearing of "show interface" counters 25w1d Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 1015 Queueing strategy: fifo Output queue: 0/40 (size/max) 5 minute input rate 3052000 bits/sec, 799 packets/sec 5 minute output rate 2943000 bits/sec, 959 packets/sec 272699111701 packets input, 390034805628930 bytes, 0 no buffer Received 12946251 broadcasts (4202505 multicasts) 0 runts, 0 giants, 0 throttles 4 input errors, 4 CRC, 0 frame, 151834 overrun, 0 ignored 0 input packets with dribble condition detected 96397195674 packets output, 38669149614929 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 unknown protocol drops 0 babbles, 0 late collision, 0 deferred 0 lost carrier, 0 no carrier 0 output buffer failures, 0 output buffers swapped out 


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