From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Sat Jul 05 2008 - 01:48:15 ART
Hi,
You should clearly understand the reason why 802.3X flowcontrol is
incompatible with MLS QoS features. A small history tour before we dig into
explanations. Originally, on half-duplex ethernet connection there was no
possibility to implement explicit flow control, since only one side could
send frames at time. (However, some may still remember that Cisco had
so-called "back-pressure" feature on some of it's switches (e.g. 1924). The
idea was that switch may send barrage of dummy frames on a half-duplex link,
effectively preventing station from transmitting information at given
moments of time).
However things changed with full-duplex point-to-point connections. The idea
of 802.3X flow-control was to allow a station on a point-to-point link to
send a special "PAUSE" frame (sent to a reserved multicast MAC address) to
signal the other end of the connection to pause sending for a certain amount
of time (the amount is specified in the frame). This simple scheme allows
for a switch to signal a station to stop overwhelming the switch and
vice-versa: station was able to tell switch to slow the sending rate. The
flowcontrol mechanics could even be used between two switches. (Note that
the similar mechanics is used in Fibre Channel switches by the virtue of
Buffer-to-Buffer flow control concept).
But then, switches evolved, and the simple idea ceased to work in scalable
and effective manner. That is, nowdays people want to have more advanced QoS
features other than simple flow-control. Modern switches are capable of
performing policing, queueing and scheduling to implement comlicated
Diff-Serv based QoS policies. Messing all that complex functionality with
simple flow control may ruin the whole thing. Imagine your switch trying to
do it best to provide expedited forwarding for CoS 5 voice frames and
suddenly recieving PAUSE frame from a station, telling it to shut up for 30
milliseconds.
This is why it's always advised to turn off 802.3X flow control whether you
enable MLS QoS on a Cisco switch be it 3550 or 3560. Note that by default
flowcontrol is *disabled* and you can only enable a switch to *receive*
PAUSE frames, but not to send them.
Use the following command to verify your current settings:
SW1#show interfaces gigabitEthernet 0/1 flowcontrol
Port Send FlowControl Receive FlowControl RxPause TxPause
admin oper admin oper
--------- -------- -------- -------- -------- ------- -------
Gi0/1 Unsupp. Unsupp. on on 0 0
HTH
-- Petr Lapukhov, CCIE #16379 (R&S/Security/SP/Voice) petr@internetworkexpert.comInternetwork Expert, Inc. http://www.InternetworkExpert.com
2008/7/5 Muhammad Imran Memon <muhammadimranmemon@yahoo.com>:
> Hi Experts, > > > I am little bit confused, As i have gone through 3550 Configuration Guide; > There is clearly written that " You must not configure both IEEE 802.x > flowcontrol and quality of service (QOS) on a switch. Before configuring > flowcontrol on an interface, use the no mls qos global configuration > command > to disable QOS on a switch. " > > But i couldn't find it any thing like that in 3560 ; that either shouldn't > they both be configured together or not; > Can i configure both on a 3560 switch. Unlike 3550 .. > > > Thanks and Regards, > Emran > > > _______________________________________________________________________ > Subscription information may be found at: > http://www.groupstudy.com/list/CCIELab.html
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