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Determining cause of Overruns+Drops on Cisco switchport

Posted on 2009-05-14
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Last Modified: 2012-06-27
We have four ports connected to an Isilon storage cluster that are getting a relatively high number of drops and the same number of overruns. Both the switch and the Isilon NIC are set to autonegotiate, and the port setup is identical to our other larger Isilon cluster that does not exhibit this issue, but has much lower bit rates on the ports. The bit rate is ten time higher on these four ports than our other cluster, so I need to determin if the ports are simply oversubscribed or if there is another issue.  Equipment is a Cisco 6513 chassis with ports on 6548-ge-tx modules.
Show interface below -

Switch#show int gig 8/13
GigabitEthernet8/13 is up, line protocol is up (connected)
  Hardware is C6k 1000Mb 802.3, address is 0017.5a28.57bc (bia 0017.5a28.57bc)
  Description: ***  Node4 ***
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Full-duplex, 1000Mb/s
  input flow-control is off, output flow-control is desired
  Clock mode is auto
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output 00:00:00, output hang never
  Last clearing of "show interface" counters 16:45:59
  Input queue: 0/2000/4893/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 7314000 bits/sec, 743 packets/sec
  5 minute output rate 1777000 bits/sec, 376 packets/sec
     56319522 packets input, 55285682936 bytes, 0 no buffer
     Received 1583 broadcasts (0 multicast)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 4893 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     47013390 packets output, 45904381346 bytes, 0 underruns
     0 output errors, 0 collisions, 0 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
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Question by:mmiller01
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LVL 32

Expert Comment

by:harbor235
ID: 24387367
Description: The number of times the receiver hardware was unable to hand received data to a hardware buffer.

Common Cause: The input rate of traffic exceeded the ability of the receiver to handle the data.

 Could have been a sustained spike in traffic? Do you have MRTG or cricket to monitir your interfaces?


harbor235 ;}
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lrmoore earned 125 total points
ID: 24403804
Your input/output 5 minute and current rates are virtually nothing on a gig link.
Broadcasts are virutally nill compared to the number of packets.
No error counters
No dropped packets
Do you have jumbo frames enabled?
Have you tried hard-setting the speed/duplex to full-duplex on both the switchport and the Isilon?
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Author Comment

by:mmiller01
ID: 24423570

I tried hard setting the speed/dup on both ends and no change.
I moved the ports to a 16 port module (6516-ge-tx) that has a different buffer structure and have not dropped a packet since. something about the 48 port module buffers that cannot hack that volume it seems.
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Assisted Solution

by:harbor235
harbor235 earned 125 total points
ID: 24424421
hmm, the volume of traffic seen via the show interface command was minimal, could also be a bad switch.

I would put the switch in a test lab and see what happens there.

good luck,

harbor235 ;}
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Author Comment

by:mmiller01
ID: 24482955
It seems moving to the 16 port modules has eliminated the drops.
Going through other ports on the 48 port modules I do not see any drops as well. Odd that something about this traffic was causing them.
At this point I can call the initial issue fixed.
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