Issue with 3750 Discards

Unanswered Question
Jul 26th, 2010

We've been experiencing for a while now issues with discarded packets on an interface on a 3750, this interface has a server attached that does backups on a nightly basis across our WAN.

When I look at both the interface on the server and the interface on the switch its showing a huge number of discards, these discards seem to follow the pattern of data throughput so as throughput increases so does the discards.

I've read some info about buffers and how these can cause discarded packets when there's none free and I know these can be tweaked but am not confident enough to know exactly what to change and by how much. I've attached the sh buffers from the switch concerned.

I'm in the process of doing a packet capture of the interface to see if the server itself is causing the issue.

If anyone can give me any advice and help it would be much appreciated?



I have this problem too.
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Nagaraja Thanthry Mon, 07/26/2010 - 07:04


From the outputs, I do not see any issue with software buffers. There are no

buffer drops. I am assuming the issue could be due to hardware buffers where

the interface does not have enough buffer space to hold the incoming data.

Please try the following:


Hold-queue 1024 in


This will increase the hold queue size for that interface. This should help

to some extent. But for a permanent solution, you might want to look at the

rate at which the server is sending that data and consider NIC teaming to

get higher bandwidth.

Hope this helps.



loworderbit Mon, 07/26/2010 - 12:47

If the issue is in fact buffer related, you may be able to implement 802.3x flow control which allows the switch to process pause frames sent by the upstream device.  This is of course contingent on the upstream device supporting the same function.

For example:



bonnardopjl Tue, 07/27/2010 - 02:09

Just check for possible duplex missmatch.

For instance, the host could be half-duplex and the switch full duplex.

jonhill Wed, 07/28/2010 - 01:04

See below output as requested.

    Transmit GigabitEthernet1/0/4            Receive

    309232361 Bytes                        945751097 Bytes

   2834901126 Unicast frames              1187858761 Unicast frames

     47789283 Multicast frames                368291 Multicast frames

     83900891 Broadcast frames               2427431 Broadcast frames

            0 Too old frames               719529300 Unicast bytes

            0 Deferred frames               63614422 Multicast bytes

            0 MTU exceeded frames          162607375 Broadcast bytes

            0 1 collision frames                   0 Alignment errors

            0 2 collision frames                   0 FCS errors

            0 3 collision frames                   0 Oversize frames

            0 4 collision frames                   0 Undersize frames

            0 5 collision frames                   0 Collision fragments

            0 6 collision frames

            0 7 collision frames          2416234081 Minimum size frames

            0 8 collision frames          4109039079 65 to 127 byte frames

            0 9 collision frames           258778270 128 to 255 byte frames

            0 10 collision frames           88140508 256 to 511 byte frames

            0 11 collision frames           10920577 512 to 1023 byte frames

            0 12 collision frames         2897476560 1024 to 1518 byte frames

            0 13 collision frames                  0 Overrun frames

            0 14 collision frames                  0 Pause frames

            0 15 collision frames

            0 Excessive collisions                 0 Symbol error frames

            0 Late collisions                      0 Invalid frames, too large

            0 VLAN discard frames                  0 Valid frames, too large

            0 Excess defer frames                  0 Invalid frames, too small

   3431800181 64 byte frames                       0 Valid frames, too small

   1927942576 127 byte frames

   2417221874 255 byte frames                      0 Too old frames

   1047801692 511 byte frames                      0 Valid oversize frames

    369587019 1023 byte frames                     0 System FCS error frames

   2362172550 1518 byte frames                     0 RxPortFifoFull drop frame

            0 Too large frames

            0 Good (1 coll) frames

            0 Good (>1 coll) frames


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