Re: Speed Mismatch?!?

Does a network infrastructure connecting devices
on 100Mbps and 1Gbps whose end to end
capacity is at least 1G bps impose any performance
limitations between the 100Mbps and 1G bps devices
other than the Max bandwdth of the slowest device.

1. TCP
Answer no.

I have here a PC on 100Mpbs and a couple of hops away
a server on 1Gbps.

The intermediate hops are all GBE or maybe a 4x
GBE Etherchannel.

##################
## 1G bps Server end
##################
C:\Documents and Settings\admin>iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 8.00 KByte (default)
------------------------------------------------------------
[1896] local 192.168.5.33 port 5001 connected with 192.168.67.249 port
2172
[ ID] Interval Transfer Bandwidth
[1896] 0.0-10.0 sec 112 MBytes 94.2 Mbits/sec

C:\Documents and Settings\admin>iperf -c 192.168.67.249
------------------------------------------------------------
Client connecting to 192.168.67.249, TCP port 5001
TCP window size: 8.00 KByte (default)
------------------------------------------------------------
[1908] local 192.168.5.33 port 2404 connected with 192.168.67.249 port
5001
[ ID] Interval Transfer Bandwidth
[1908] 0.0-10.0 sec 110 MBytes 92.0 Mbits/sec



#################
## 100Mbps PC end.
#################
M:\Documents and Settings\admin>iperf -c 192.168.5.33
------------------------------------------------------------
Client connecting to 192.168.5.33, TCP port 5001
TCP window size: 63.0 KByte (default)
------------------------------------------------------------
[1704] local 192.168.67.249 port 2172 connected with 192.168.5.33 port
5001
[ ID] Interval Transfer Bandwidth
[1704] 0.0-10.0 sec 112 MBytes 94.1 Mbits/sec


M:\Documents and Settings\admin>iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 8.00 KByte (default)
------------------------------------------------------------
[1668] local 192.168.67.249 port 5001 connected with 192.168.5.33 port
2404
[ ID] Interval Transfer Bandwidth
[1668] 0.0-10.0 sec 110 MBytes 92.1 Mbits/sec

This works in this case. No tuning, no optimisation
just perfect performnce right out of the box.

If it didn't work we would be in BIG trouble. All of our servers
are on GBE and a lot of the clients are on FE.

Millions and millions of people are using just such a
setup as you describe.

OK, yours isn't working.

Maybe this is the clue?
Also, in addition to the TCP re-Trans frames, the Cisco switch (when I
put in the patch between these hosts) is telling me I've got "late
collisions" on the gbit interface during the slow transfers. WTF?
The PC's are all 100/full, in auto. Everything is full duplex, so
why am I seeing ANY collisions of any kind at all???

"Late collisions" are your problem, or at least one of them.

They are caused by two things in modern switched networks.
Firstly collisions can only occur on Half Duplex ports.

Late collisions are a consequence of a duplex missmatch.
They occur on the HD end since the FD end just transmitts
when it feels like it and sometimes there is an out of window
collision.

I have found (a while back) that WIndows drivers can report
duplex settings incorrectly. In dicfficult cases I trust
ONLY the port counters on Cisco networking kit.

Use the counters to determine if you have a duplex missmatch.

If there is a missmatch AND there is enough traffic
(let's say) there WILL BE late collisions on the HD end.

The FD end will be seeing CRC and Align errors.

So, very carefully determine what to do to emiminate
the duplex missmatch and then roll it out onto the
various kit.

Pick a couple of ports on one switch set up 1000 on
one and 100 on another.

Clear the counters

run some traffic if the performance is poor send

sh run int fa xx
sh run int gi xx

sh int fa xx
sh int gi xx

If you download iperf and use it to create traffic in
both directions simultaneously you will for sure
show up any problems.

For some reason when I do both directions simultaneously I
get only 90Mbps one way and 25Mbps he other way.
I don't know why but I think that if it was a duplex missmatch
it would be worse.

.

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