Output drops on Gig uplink to SonicWALL

[DuncanM2008]

Hello,

I'm currently trying to diagnose a re-occuring problem with output drops on an uplink interface to a SonicWALL firewall, between 1am - 3am every night we receive alerts that the output drops (discards) counter has incremented.

This time frame happens to be when several network based backups are running so that's probably to blame for the added traffic count but I'm trying to narrow down what's actually dropping the traffic.

The switch hardware is a WS-C3750X-48 running C3750E-UNIVERSALK9-M , 12.2 (55) SE3.

My current thinking is that the output drops are indicative of congestion on the link to the SonicWALL indicating that the SonicWALL unit is incapable of processing the traffic flow fast enough thus causing the congestion and ultimately the output drops on the Gi2/0/48 interface.

According to SonicWALL the unit is rated at 1.5Gbps throughput without UTM services (pure stateful), and no UTM services are enabled.

I've posted a section of the show interface command:

  Keepalive set (10 sec)

  Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX

  input flow-control is off, output flow-control is unsupported

  ARP type: ARPA, ARP Timeout 04:00:00

  Last input never, output 00:00:53, output hang never

  Last clearing of "show interface" counters 3d23h

  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 411143

  Queueing strategy: fifo

  Output queue: 0/40 (size/max)

Has anyone seen this before?

Thanks,

Duncan.

[paolo bevilacqua]

Do you have any QoS configured on the switch? if yes, remove it and you will have no more drops.

Note: the switch cannot detect if/when the device attached is unable to process traffic:

  input flow-control is off, output flow-control is unsupported

consequently, drops are not caused by the attached device, or its performances.

[DuncanM2008]

Thanks for your prompt response, there is currently no QoS policies configured on the switch. The switch configuration is in essence very simple 96 host facing ports with about 10 etherchannel groups and two SVI's doing inter-vlan routing.

Am I looking at a genuine bug / glitch then?

Thanks,

[Joseph W. Doherty]

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Posting

Since flow control is not enabled, your drops aren't due to the performance of your firewall, but to more than a gig being sent to the egress interface.  If you have more than the egress port's bandwidth available for ingress to the switch, and actual ingress offered rate is higher than the egress available transmission rate, excess traffic will be queued; when queue fills, packets are dropped.