3750G dropping packets

Difan Zhao · ‎01-17-2014

Hi expert,

I have a switch stack made of two WS-C3750G-24TS-1U. I was reported with performance issue so I was checking interface for drops. On the interface there is no drop and the traffic is fairly small (72mbps on 1Gbps link)

#sho int g1/0/23

GigabitEthernet1/0/23 is up, line protocol is up (connected)

Hardware is Gigabit Ethernet, address is 0014.a906.5217 (bia 0014.a906.5217)

Description: DI NAS Storage

MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

reliability 255/255, txload 18/255, rxload 10/255

Encapsulation ARPA, loopback not set

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 00:00:16, output 00:00:00, output hang never

Last clearing of "show interface" counters 7w1d

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

Queueing strategy: fifo

Output queue: 0/40 (size/max)

30 second input rate 40518000 bits/sec, 4390 packets/sec

30 second output rate 71560000 bits/sec, 9342 packets/sec

914641996 packets input, 3831824985 bytes, 0 no buffer

Received 174840 broadcasts (0 multicast)

0 runts, 0 giants, 0 throttles

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

0 watchdog, 147298 multicast, 0 pause input

0 input packets with dribble condition detected

1309563891 packets output, 980418609 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

However if I use another command I see drop on the ASIC

#show platform port-asic stats drop g1/0/23

Interface Gi1/0/23 TxQueue Drop Statistics

Queue 0

Weight 0 Frames 0

Weight 1 Frames 0

Weight 2 Frames 0

Queue 1

Weight 0 Frames 0

Weight 1 Frames 0

Weight 2 Frames 0

Queue 2

Weight 0 Frames 0

Weight 1 Frames 0

Weight 2 Frames 0

Queue 3

Weight 0 Frames 0

Weight 1 Frames 0

Weight 2 Frames 3414434

On the switch I don't have MLS QOS enabled.

#show mls qos

QoS is disabled

QoS ip packet dscp rewrite is enabled

I googled and some said that it could be too small tx-buffer on the interface but the "no-buffer" counter is also 0... Any ideas why this is happening? Any direction on how to do this type of slowness troubleshooting is highly appreciated!

Thanks!

Leo Laohoo · ‎01-17-2014

I was reported with performance issue so I was checking interface for drops.

What you are seeing is NORMAL. Why?

Because the 3750 was NEVER designed to be used for high-speed servers like NAS arrays. All Catalyst switches (2K and 3K) have very low memory buffer so a continuous traffic to/from a NAS will overflow this buffer and the switch will start dropping packets. Exactly what you are seeing.

Cisco's Nexus solution is built around DC and storage.

Joseph W. Doherty · ‎01-17-2014

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Posting

A Leo has described, the problem with 3750s is their buffers don't support large transient buffering.

The later 3750X series is documented at only having 2 MB of buffering for each set of 24 ports and for the uplink ports. (Unknown if same applies to original series.)

What IOS are you using? If using a version before 12.2(55)SE8, there might be some benefit to upgrading.

As also noted by Leo, the Nexus series are designed for data center type hosts, but you might also find a 4900 series to work much better too.

Difan Zhao · ‎01-20-2014

Thanks guys.

Leo, before I ask customer to buy new equipments, I have to prove that it is the capacity limit on the 3750. we all think that it is the buffer. However in my "show interface" command the "no buffer" counter is 0. Why is that? Is there a way to tell for sure that it is the low memory buffer problem?

Hi Joseph, I will suggest a upgrade. Right now it is very low version, something like 12.2(25)

Thanks!

Leo Laohoo · ‎01-20-2014

Leo, before I ask customer to buy new equipments, I have to prove that it is the capacity limit on the 3750. we all think that it is the buffer. However in my "show interface" command the "no buffer" counter is 0. Why is that? Is there a way to tell for sure that it is the low memory buffer problem?

Talk to your local Cisco SE/AM. Get them to give your customer a Nexus or, as Joseph has noted, Catalyst 4900 presentation.