3750 - Total output drops - Issue

parm UK · ‎07-29-2015

Hi Guys,

Im hoping for some assistance.

We own a small building rented out as managed office space. As part of this we have a 1Gb internet feed which we then distribute throughout the building. With each customer / office having their own managed firewall. Connectivity is sold as either 10Mb or 100Mb.

So we have a 1Gb internet feed out from our ISP router (rj45) into a 3750. To which the managed firewall is attached, port speed is set manually to 10 or 100 depending on require connectivity as above.

The 3750 is completly flat, no vlans, QoS disabed (default) - no duplex issues, etc

on certain interfaces i'm seeing output drops as per;

GigabitEthernet1/0/8 is up, line protocol is up (connected)

Hardware is Gigabit Ethernet, address is acf2.c501.7688 (bia acf2.c501.7688)

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

reliability 255/255, txload 3/255, 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 1y18w, output 00:00:00, output hang never

Last clearing of "show interface" counters 20:57:21

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 20156

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 327000 bits/sec, 184 packets/sec

5 minute output rate 1409000 bits/sec, 204 packets/sec

6280347 packets input, 1668073759 bytes, 0 no buffer

Received 2520 broadcasts (0 multicasts)

0 runts, 0 giants, 0 throttles

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

0 watchdog, 0 multicast, 0 pause input

0 input packets with dribble condition detected

9141511 packets output, 9402986888 bytes, 0 underruns

0 output errors, 0 collisions, 0 interface resets

0 unknown protocol drops

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

QoS is disabled as per

NET#sh mls qos
QoS is disabled
QoS ip packet dscp rewrite is enabled

Hence the drops are not due to this. I know 3750 has small buffers however we thought it could cope with this. ?

is the 3750 not up to the job?

I know this may be caused my microbursts - but do these not effect LAN switchs? I thought as this switch site betwwen ISP router and customer firewalls this should nto be the case.

Any help appreciated.

Thanks in Advance

ParmbUK

Joseph W. Doherty · ‎07-30-2015

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It's likely, as you've already supposed, the 3750's small buffers are getting exhausted during some burst. (Microbursts also apply to LANs.)

Disabling QoS, on a 3750, avoids queuing resource issues caused by default QoS settings, but I've found, you can sometimes dramatically decrease drops, by enabling queuing and tuning the buffer settings. (I've found tuning the 3750 to work more from its common pool, rather than reserved interface pools, does the trick if not all ports are busy at the same time.)

The 3750 provides 2 MB of buffer RAM per 24 edge ports and for its uplink ports, so how you place busy ports can be important too.