05-18-2012 03:05 PM - edited 03-07-2019 06:47 AM
Welcome to the Cisco Support Community Ask the Expert conversation. This is an opportunity to learn from Cisco experts Shashank Singh and Sweta Morga about implementation and working and troubleshooting QoS on Cisco Catalyst 2960, 3650, 3750, 4500 and 6500 switches.
Shashank Singh graduated in 2009 with a bachelor's degree in Computer Science and Engineering from VIT University, Vellore India. Prior to joining Cisco he worked at General Electric as a software engineer. Later on he joined the Cisco Technical Assistance Center as an engineer in October of 2009. He has been working on LAN Switching technologies in TAC since then. Shashank also holds a CCNP certificate. QoS on Catalyst switches is one of the areas of his interest.
Sweta Mogra is a Computer Science & Engineering graduate from VIT University, India. She has worked as a consultant with Tata Consultancy Services before joining Cisco's Technical Assistance Center (TAC) in 2011. She is currently working on LAN Switching technologies and QoS as one of her areas of expertise.
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Shashank and Sweta might not be able to answer each question due to the volume expected during this event. Remember that you can continue the conversation on the Network Infastructure sub-communityLan Switching forum shortly after the event. This event lasts through June 1, 2012. Visit this forum often to view responses to your questions and the questions of other community members.
05-23-2012 06:48 AM
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You need to consider bandwidth ratios for the queues. If bandwidth ratios were the same, then they would be treated the same. If bandwidth ratios were different, scheduler will remove lesser number of packets from queue having lower bandwidth guarantee.
So if I want to have more packets in queue 1 dropped than queue 2 I need to have the bandwidth for queue 1 higher than queue 2 - am I understanding you correctly?
05-23-2012 08:12 AM
Packets in queue with lower bandwidth guarantee are more likely to be dropped. so, queue 1 needs to have lower bandwidth to drop more packets.
05-22-2012 07:46 PM
Shashank,
In my limited understanding of dscp and cos, isn't dscp used at layer 3 and cost at layer 2?
Regards,
Jason
05-22-2012 07:54 PM
You are right Jason. Dscp is derived from the TOS field in IP header (layer 3) and CoS is a field in the 802.1Q tag (layer 2) in ethernet frame.
Regards,
Shashank
05-23-2012 02:52 AM
I am trying to convert the smartport profile on a CE 500 switch to a 2960 switch.
Can anyone help ?
05-23-2012 08:15 AM
Hi Lewis,
The smartport roles on CE500 are based on the type of devices to be connected to the switch ports. If you had Cisco IP phones role for a port on CE500, in that case you can use auto-qos feature on 2960 which suffice most of the times. Or if you are connecting to other switch and your traffic is coming premarked at the source, you can simply trust the marking (mls qos trust cos/dscp). If however you are planning to mark traffic on the switch, we will have to apply the service policy.
-Sweta
05-23-2012 06:59 AM
On my 3750 switches, I can force the cos for all inbound traffic from the connected device with the following commands:
MLS qos cos 5
MLS qos cost override
What's the equivalent set of commands to override the cost values on a 6500 switch? I am running IOS on both sets of devices.
Regards,
Jason
05-23-2012 09:27 AM
Hi Jason,
The override suffix is not supported on 6500 but you can have an alternative configuration for that. Try configuring a policy map that matches the ingress traffic followed by using set command to configure the desired cos value.
Sample config below:
access-list 10 permit any
Class Map match-all TEST
Match access-group 10
policy-map COS
class TEST
set cos 5
Regards,
Sweta
05-23-2012 08:34 AM
I have a question on 3560/3750 threshold counters.
This is the default DSCP output threshold map, from the 'show mls qos map' command:
Dscp-outputq-threshold map:
d1 :d2 0 1 2 3 4 5 6 7 8 9
------------------------------------------------------------
0 : 02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01 02-01
1 : 02-01 02-01 02-01 02-01 02-01 02-01 03-01 03-01 03-01 03-01
2 : 03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01 03-01
3 : 03-01 03-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01
4 : 01-01 01-01 01-01 01-01 01-01 01-01 01-01 01-01 04-01 04-01
5 : 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01 04-01
6 : 04-01 04-01 04-01 04-01
All traffic is mapped to Threshold 1 in their respective queues.
However, when looking at the 'show mls qos int x/y stat' command, it shows extensive packets in the the other thresholds. Particularly queue 3, threshold 3:
output queues enqueued:
queue: threshold1 threshold2 threshold3
-----------------------------------------------
queue 0: 0 0 0
queue 1: 675 0 24
queue 2: 1 0 0
queue 3: 80 0 1848
Why?
05-23-2012 10:12 AM
Please let me know the following:
Is QoS enabled on the switch? (sh mls qos)
Is the counter against q3t3 incrementing as of now? Is it possible that this is stale data?
Is is possible that cos-output- map maps some markings to q3t3? Please check "sh mls qos maps cos-out". If not please share the following details:
sh ver
sh mls qos queue-set
Regards,
Shashank
05-23-2012 03:37 PM
Hi,
Yes, QoS is enabled on the switch, and there is a fairly extensive port based marking/policing policy active on the edge ports, modeled on the Medianet 4.0 guidelines.
The cos-map is default as well:
Cos-outputq-threshold map:
cos: 0 1 2 3 4 5 6 7
------------------------------------
queue-threshold: 2-1 2-1 3-1 3-1 4-1 1-1 4-1 4-1
As is the queue-set:
Queueset: 1
Queue : 1 2 3 4
----------------------------------------------
buffers : 25 25 25 25
threshold1: 100 200 100 100
threshold2: 100 200 100 100
reserved : 50 50 50 50
maximum : 400 400 400 400
This is a 3750X stack running 12.2(53)SE2 code.
This switch may have had some auto-qos config on it at one time, which has since been removed, although not rebooted. Could the counters still be reporting from old config, even though all queueing has been defaulted?
05-23-2012 11:48 PM
I would believe that the counters date back to the time before reboot. Please see if you can reboot the switch and check again.
Regards,
Shashank
05-23-2012 08:54 AM
I have an installation where the switch access port faces a phone (7942 or 7962) with a subtended PC.
The PC is also running a Cisco softphone (IP Communicator).
The switch config has the option of programming the port to trust either a phone OR a softphone
Switch(config-if)#auto qos voip ?
cisco-phone Trust the QoS marking of Cisco IP Phone
cisco-softphone Trust the QoS marking of Cisco IP SoftPhone
trust Trust the DSCP/CoS marking
I know that the switch uses CDP to recognise either in order to trust it.
I like the idea of CDP checking for a trusted source of QOS markings.
How can I make the port work with both a cisco-phone AND a cisco-softphone, without just trusting all DSCP marking (like from other apps on the PC)?
Or asking another way,
05-23-2012 10:01 AM
Hi,
Unfortunately we cannot implement auto-qos for IP phones and soft-phones together on an edge port. If you don't want to trust the ingress traffic using trust statement, we are only left with the option of using MQC (class-maps). You can create class maps to classify the two types of traffic received followed by mapping them to different queues.
Regards,
Sweta
05-23-2012 10:41 AM
Thanks Sweta for clearing that up.
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