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Troubleshooting Interface Drops

Introduction

Output drops are a result of the traffic rate exceeding the maximum bandwidth specification of a given interface. Given that this is normally an outcome to interface congestion the following steps explain the commands used to clarify the total interface usage in both terms of Mbits and overall utilization.

Output Drop Totals

To confirm the total drops of a particular port the following command is run:

switch#sh int fa0/1 | in drop|bits
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 142478
5 minute input rate 1079000 bits/sec, 5524 packets/sec
5 minute output rate 8688000 bits/sec, 9018 packets/sec

 We can also see the interface errors using the following command. The output errors are report here as OutDiscards.

switch#show interface FastEthernet0/1 counters errors
Port        Align-Err     FCS-Err    Xmit-Err     Rcv-Err  UnderSize  OutDiscards
Fa0/1               0           0           0           0          0       179107

Port      Single-Col  Multi-Col   Late-Col  Excess-Col  Carri-Sen      Runts     Giants
Fa0/1              0          0          0           0          0          0          0

Changing Load-Intervals

As you can see from the previous section the load-intervals are configured to 5 minute intervals. To obtain a clearer view of the total throughput of a given interface the load-interval can be reduced to a lower value.

Below we change the load-interval to 30 secs:

switch(config)#interface FastEthernet0/1
switch(config-if)#load-interval 30

switch#sh int fa0/1 | in drop|bits
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 142478
30 second input rate 10079000 bits/sec, 5524 packets/sec
30 second output rate 86888000 bits/sec, 9018 packets/sec

Interface Ultization

To view the overall utilization (%) of an interface the following command is run:

switch#sh controllers fastEthernet 0/1 utilization
Receive Bandwidth Percentage Utilization   : 13
Transmit Bandwidth Percentage Utilization  : 100

Additional Errors

To display further details of the traffic that has transversed the interface the following command is run :

switch#sh controllers ethernet-controller fastEthernet 0/1    
Transmit FastEthernet0/1                 Receive
   1755650038 Bytes                        550748297 Bytes
    565241190 Unicast frames              3235647489 Unicast frames
     19304109 Multicast frames                     0 Multicast frames
      3784939 Broadcast frames                   671 Broadcast frames
            0 Too old frames               550705353 Unicast bytes
            0 Deferred frames                      0 Multicast bytes
            0 MTU exceeded frames              42944 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          1912867443 Minimum size frames
            0 8 collision frames          3138104280 65 to 127 byte frames
            0 9 collision frames            80595969 128 to 255 byte frames
            0 10 collision frames           29329601 256 to 511 byte frames
            0 11 collision frames          551146604 512 to 1023 byte frames
            0 12 collision frames         1818571559 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
   2417481382 64 byte frames                       0 Valid frames, too small
   1021375875 127 byte frames
    691455767 255 byte frames                      0 Too old frames
    562825073 511 byte frames                      0 Valid oversize frames
    292399558 1023 byte frames                     0 System FCS error frames
   4192727175 1518 byte frames                     0 RxPortFifoFull drop frame
            0 Too large frames
            0 Good (1 coll) frames
            0 Good (>1 coll) frames 

Options

Unfortunately there are only 2 solutions to output drops. These are :

  1. Increase the interface speed.
  2. Reduce the traffic throughput.

Reference

http://www.cisco.com/en/US/products/hw/routers/ps133/products_tech_note09186a0080094791.shtml

Tags: Cisco

About the Author

RDonato

R Donato

Rick Donato is the Founder and Chief Editor of Fir3net.com. He currently works as a Principal Network Security Engineer and has a keen interest in automation and the cloud.

You can find Rick on Twitter @f3lix001