Output Drops on a switch

Unanswered Question
Dec 31st, 2008

Hi,

A few days ago, I started getting severe output drops on a switch (3750G). Going back and looking at the configs in CVS, nothing has been changed. The port in question is a gig port, connected to a 2960. No errors are being reported on the 2960. I'm thinking some device may have been introduced onto the network causing these output drops to occur, but I'm not sure how to debug it. The port in question is configured like this:

interface GigabitEthernet2/0/24

description Link to 2960

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 333,555

switchport mode trunk

switchport nonegotiate

no cdp enable

#sh interface GigabitEthernet2/0/24

GigabitEthernet2/0/24 is up, line protocol is up (connected)

Hardware is Gigabit Ethernet, address is 001e.14d8.2718 (bia 001e.14d8.2718)

Description: Link to 2960

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full-duplex, 100Mb/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 never, output hang never

Last clearing of "show interface" counters 01:21:36

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 198102

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 0 bits/sec, 0 packets/sec

5 minute output rate 0 bits/sec, 0 packets/sec

10422 packets input, 840624 bytes, 0 no buffer

Received 7129 broadcasts (4006 multicasts)

0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog, 4006 multicast, 0 pause input

0 input packets with dribble condition detected

30006 packets output, 2482275 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

What's interesting is when I watch the interface, it will say Total output drops: 198102, then say Total output drops: 99051, then say Total output drops: 0, then start the process all over again.

If this were a normal access port, I'd start by looking at the device attached to it, but this is a trunk. Is there any way to determine what could be causing these drops to spike?

Thanks for any help!

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John Blakley Wed, 12/31/2008 - 11:48

If you can put a sniffer on a port and create a span session, you may be able to see what's going on.

HTH,

John

MATTHEW BECK Wed, 12/31/2008 - 11:54

Hi,

If a port is dropping that much traffic in the outbound direction, I would look upstream for incoming traffic spikes. Do you have pps and bw input metrics for all ports coming into VLAN 333 and 555 on the 3750? Do any of those ports show an increase in input traffic compared to before the output drops started? Does the 3750 link up to core switch where there are other sources in VLAN 333 and 555? Are there multicast sources in those VLANs and is PIM/IGMP configured correctly to contain it?

Good luck and Happy New Year!

Matt

glen.grant Wed, 12/31/2008 - 13:21

Also make sure the 2960 end is set as "auto" for speed and duplex as the 3750 appears to be auto . Verify with "show int status" . Sometimes if you get brief spikes of between 90 and 100 % of bandwidth that will also cause output drops . Look at the utilization on both sides when the drops appear to be increasing with the show controllers utilization " command.

robertpatrick Wed, 12/31/2008 - 15:14

Looks like you have a 3750G with Gigabit Ethernet interface connected to a 2960 with a Fast Ethernet interface. Are any of the other 3750G interfaces operating at 1 Gbps (1000 Mbps) or a combination of multiple interfaces on the 3750G capable of carrying >100 Mbps of traffic in the two VLANs you are trunking to the 2960? The 2960 can only accept 100 Mpbs of traffic (your show int output states the interface is running 100 Mbps). If one or more hosts (e.g. combination of multiple hosts within the two VLANs you are trunking) on the 3750G or further upstream are transmitting at very fast rates (>100 Mbps), you will get output discards on the link to the 2960.

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