3750G speed 100 & srr-queue bandwidth limit 50 failing

[MARK CASEY]

Hi

I have a live system which is failing

I have set up a test system replicating the live system, consisting of 2 x 3750G's.

A 3750G (Remote) connected to another 3750G (Host) with the following configuration

All switches are Version 12.2(55)SE12

interface GigabitEthernet1/0/1
description MSA Uplink
switchport trunk encapsulation dot1q
switchport trunk native vlan 5
switchport trunk allowed vlan 99,116
switchport mode trunk
speed 100
srr-queue bandwidth share 1 25 70 5
srr-queue bandwidth limit 50
queue-set 2
priority-queue out
mls qos trust dscp
macro description cisco-router

I am doing a speed test using Iperf 3 between 2 hosts on either side of the system using vlan 116

Remote (windows) Host (Linux)

using iperf3 -c 10.245.23.41 -u -b40

TEST 1

If I do the test Host -> Remote is see 40 Mbit transfer rate - No packets dropped on any interface

If I do the test Remote -> Host is see 12 Mbit transfer rate  and matching packets dropped on the interface GigabitEthernet1/0/1

The packet loss is associated with DSCP 0 in

sh mls qos interface gigabitEthernet 1/0/1 statistics

Yes there is all sorts of other potential issues & settings but wait...

TEST2

But if i take the same Remote configuration and put it on a 3750 so interface 1 becomes

interface FastEthernet1/0/1
description MSA Uplink
switchport trunk encapsulation dot1q
switchport trunk native vlan 5
switchport trunk allowed vlan 99,116
switchport mode trunk
srr-queue bandwidth share 1 25 70 5
srr-queue bandwidth limit 50
queue-set 2
priority-queue out
mls qos trust dscp
macro description cisco-router

I see 40 Mbps in both directions.

TEST 3

On the original setup with 2 x 3750 G switches

If i leave the interfaces at GBit and send 400M I see no packet loss

CONCLUSIONS

My switch configuration in TEST1 is valid as it works with TEST2 so my mls qos, buffers etc configuration are ok, because TEST 2 works

As TEST3 works, it seems like the SRR functions don't like the speed 100

Remote config attached

Anyone got any ideas how to fix this so I can get speed 100 and SRR woorking in the live system  ?

Thanks in Advance

Mark

[pieterh]

if you use "speed 100 " on the 3750G's then "automagically" also duplex negotiation is disabled
-> this may result in a duplex mismatch with the remote side (which is still set to auto?), hence the difference in throughput
on the normal 3750 you do not use speed 100 so there is no duplex mismatch.; the same on two 3750G's

=> use auto/auto or speed+duplex on both sides of the link

[Joseph W. Doherty]

Assuming this issue cannot be explained by a duplex mismatch, as PieterH suggests, I recall (?) many years ago (before 3750Es), someone else had a "curious" 3750 performance issue, when mixing running ports at gig and FE (more than the obvious of gig congesting a FE port).

We didn't find a cause, but issue wasn't critical to poster either.

Without getting into the many 3750 buffer management issues when QoS is enabled (I presume it is, based on the interface commands configured), I'm also wondering about the usage of "bandwidth limit 50". I presume you know that this command is not "exact", but can we presume it really works alike on a gig interface running at 100, vs. a FE port? (I've always presumed we would get the same result, but I wonder if that's true.)

For "giggles", remove that command, from the interfaces, and try again. (It shouldn't be needed, in a lab test, where you're only transmitting at 40 Mbps.)