From: ccie2be (ccie2be@nyc.rr.com)
Date: Fri Apr 08 2005 - 10:56:20 GMT-3
Hi Simon,
Actually, according to Wendell Odom, you can have 2 priority queues. I
don't remember all the details or how scheduling is done between the 2
priority queues but it should work.
I've never tried it, but I wonder what happens when 2 priority queues are
configured and then 3 priority queues are configured.
Would IOS take the 3rd priority queue command or would it give you an error
message?
It's interesting to think about but probably not that relevant in terms of
the lab.
Tim
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
simon hart
Sent: Friday, April 08, 2005 7:48 AM
To: steve.skinner@uk.pwc.com; ccielab@groupstudy.com
Subject: RE: priority command in MDQ
Steve,
With CBWFQ/LLQ, the order by which you enter the classes/queues does not
dictate the manner by which the traffic is scheduled, with the exception of
the priorirty queue,
With CBWFQ without a priority queue and under congestion conditions each
queue is 'dequeued' to the configured bandwidth command. The exact manner
by which the scheduler takes this packets and puts them onto the TX-ring of
the interface is proprietary and has not been published by Cisco, therefore
we can only guess at this particular scheduling for each queue ( I expect
the manner by which each queue is serviced is similar to WRR).
Now by configuring a Priority queue, you are changing the scheduling of the
CBWFQ somewhat. The Priority queue will get serviced first, if their are
packets in its queue, up to it's configured bandwidth. Once the Priority
queue has been dealt with the scheduler will now service the other queues in
the manner described previously.
This type of queueing is important for latency sensitive applications such
as Voice or Video.
A couple of points on the priority queue,
1. You can only have one priority queue (you configuration is incorrect in
this regard.)
2. The priority queue should only represent 30% of the Max Reserved
Bandwidth of the Interface. You can go higher, however you can run into
problems, it is not unknown for the CPU to sky rocket under such conditions.
3. You should only put one type of traffic in the Priority queue. If you
are trying to deliver a Voice service (under an SLA), then you want to make
sure that the voice is behaving as expected, if you mix it up with other
traffic (such as Video), you cannot guarentee latency and jitter of the
voice traffic.
4. The priority command does not define a strict priority for scheduling
each class.
Your Policy below will be doing this
VOIP will be policed at 1Mbps and conforming traffic marked as IPprec 4
(Flash Override)
Backup Servers will be policed at 250Kbps
All other traffic will be marked with IPprec 5 (Critical)
Under congestion conditions your VOIP will be guaranteed 1Mbps and this
1Mbps will be dequeued before any other traffic.
Your other two queues do not have any bandwidth configured, therefore under
congestion conditions the BackupServer and Class Default queue will compete
for bandwidth. However under these conditions the BackupServer will only
'compete' up to its policed rate. I would suggest entering a bandwidth
command on the BackupServer class in order to guarentee its policed rate.
Your show command shows that you have one strict priority queue, that will
be (i think but needs confirmation) treated as Conversation 264 on the
ouptut scheduler and that this particular queue is policed at 1Mbps and will
bc at 25K bytes, therefore the Tc for the priority queue is 200ms.
HTH
Simon
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
steve.skinner@uk.pwc.com
Sent: 08 April 2005 11:36
To: ccielab@groupstudy.com
Subject: priority command in MDQ
guys .
i have this config
i am trying to understand the prioirty command within the class .
according to my understading ,the prioity command sets a Strict Priority
per class in the policy .
but as i thought you would usually only have 1 type of traffic per class
,as in this case ,why do i need to prioritise the only traffic type in this
class .
i have put my most important class at the top of the policy ,so this will
get queued first . but other than that i didn`t think i would need to do
anything else.
TIA
steve
class-map match-any Steve_VOIP
match access-group 90
class-map match-any Backup_Servers
match access-group 111
!
policy-map Siemens_Trunk
description 1000Kbs Class 1 voice policy
class Steve_VOIP
police cir 1000000
conform-action set-prec-transmit 4
exceed-action drop
priority 1000
class Backup_Servers
police cir 250000
exceed-action drop
class class-default
prioity 100
set ip precedence 5
!
output from policy
Strict Priority
Output Queue: Conversation 264
Bandwidth 1000 (kbps) Burst 25000 (Bytes)
This archive was generated by hypermail 2.1.4 : Tue May 03 2005 - 07:54:55 GMT-3