Ask The Expert:QoS on Catalyst Switches - Page 2

ciscomoderator · ‎08-12-2011

With Shashank Singh

Welcome to the Cisco Support Community Ask the Expert conversation. This is an opportunity to get an update on QoS on Catalyst 2960, 3550, 3560, 3750, 4500 and 6500 series switches with Cisco expert Shashank Singh. Shashank 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.

Remember to use the rating system to let Shashank know if you have received an adequate response.

Shashank 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 InfrastructureLAN Switching discussion forum shortly after the event. This event lasts through August 26 , 2011. Visit this forum often to view responses to your questions and the questions of other community members.

Dipesh Patel · ‎08-15-2011

Dear Shashank,

I have to desgn and implement QOS for Campus Network.

Existing Scenario : To understand the existing Topology, I have attached diagram with the devices Make and model.

On WAN in MPLS clound the QOS has been configured by MPLS Service Provider i.e.

COS1 : 20% of BW --- Voice

COS2 : 20% of BW --- Video

COS3 : 25% of BW --- SAP

COS4 : 35% of BW --- Default

Main Traffic in Network :

IP Telephony : AVAYA CM 6.0 (Avaya Aura Technology)
Video Conferencing : Polycom
SAP / ERP Application
SMTP/ MS Exchange Traffic
Internet Traffic
Microsoft-ds traffic
CIFS traffic between Servers
And other default Traffic

Problem : Voice and Video Clarity / Quality is not good.

Requirement : To implement QOS in LAN and WAN.

I have gone through the documentation regarding the implementation of QOS for LAN. But it's littla bit confusing.

There are several queries if you will solve, it will be a great help me to go ahead.

Queries :

Should we go with Auto QOS or Manually configuration of QOS?
If AutoQos (SRND v4) than On Access Layer Switch,
- On Disctribution switch :
  - Which command I should apply on the Trunk Ports?

On CORE switch :
- Which command I should apply on trunk ports and on ports connected to Router?

In AutoQOS SRNDv4 I have seen 4 types of cmds are there where should I use? :
- auto qos classify {police}

Post Auto QOS configuration - How to Tune the same ?

If no Auto QOS than how to configure QOS at Access layer / Distribution layer / CORE layer? Management insterested to go with this option. Please help. I m novice for this.

If you can provide the sample configuration for Access layer, Distribution lalyer and CORE layer according to the attached topology than it will be a great help for me.

I have gone throguh many documents on the Cisco Web. But Lots of Confusion !

In addition to this, After configuration of the QOS How can I monitor the QOS? Is there any Tool to monitor the same.

Waiting for the Replies from you.

Topology :

Thanks in Advance

Regards

Shashank Singh · ‎08-15-2011

Hi Dipesh,

Please find the answers inline:

Queries :

Should we go with Auto QOS or Manually configuration of QOS?

Auto Qos assumes that the traffic on the ingress is premarked and we need to trust that marking to apply QoS. Again, 'auto qos voip' is designed only for the access ports that directly connect the traffic source (say an IP phone). If traffic is premarked and you do not want to alter the marking, you can go ahead and apply auto qos voip trust on the ingress access port.

If AutoQos (SRND v4) than On Access Layer Switch,

On Disctribution switch :

Which command I should apply on the Trunk Ports?

On CORE switch :

Which command I should apply on trunk ports and on ports connected to Router?

You can go ahead and trust the marking on the trunk ports (mls qos trust cos|dscp) on all subsequent access and core switches. This will make the switch queue traffic according to the marking.

There are four egress queues per interface on 2960/3750/3560 switches. Each marking goes to one particular queue. You can check the individual buffers using 'sh mls qos queue-set' on these switches. The buffers generally need not be tweaked unless we are sure that they are insufficient for the amount of traffic present for that queue. Each platform has its own port basd QoS commands which are available in detail in their respective configuration guides.

In AutoQOS SRNDv4 I have seen 4 types of cmds are there where should I use? : auto qos classify {police}

If you enter the auto qos classify command, the switch automatically creates class maps and policy maps. This is a template and saves you from the pain of configuring the policy maps manually. Note that these policies classify and remark the traffic as per the best practices.

Post Auto QOS configuration - How to Tune the same ?

This will require some analysis and traffic engineering. Once you apply auto qos switch uses a template to classify and mark traffic, and queues it accordingly. However, it may happen that the queue resources (buffers/bandwidth) may be insifficient for one/more queues. This may show up as output drops on some interfaces. If you see this, you may need to troubleshoot further and find out the queue that is being overutilized and may be tune it to suit your traffic.

If no Auto QOS than how to configure QOS at Access layer / Distribution layer / CORE layer? Management insterested to go with this option. Please help. I m novice for this.

When it comes to QoS, there is no standard configuration that always works. Auto Qos uses the default best values to apply QoS but it may still not suit your network. Hence, to come up with manual configurations, it is important to know whether or not you have congestion in your links. If you have congestion, you will need to find out the amount of traffic for each type and which traffic is lesser important than others and can be dropped.

You can go through the borderless campus 1.0 design guide for recommended configuration to get started.

http://www.cisco.com/en/US/docs/solutions/Enterprise/Campus/Borderless_Campus_Network_1.0/BN_Campus_QoS.html

Also, for a detailed Auto Qos overview, you may go through the whitepaper.

http://www.cisco.com/en/US/tech/tk543/tk759/technologies_white_paper09186a00801348bc.shtml

If you have any specific questions, feel free to let me know.

Cheers,

Shashank

Dipesh Patel · ‎08-16-2011

Dear Shashank,

Thanks for the Reply.

We have a Voice network of Avaya CM 6.0 with S8700. And Video Conferencing Devices are of Polycom.

Is it ok if we will go with Initially:

1. All access Ports with switch---IP Phone----PC/Laptop as *** auto-qos voip trust ***.

2. All Access ports with Printers,Servers without Voice Traffic with *** auto-qos clasify police ***

3. All the trunk ports on Access, Distribution and CORE switch with *** auto-qos voip trust ***

4. Port connected to Router with *** auto-qos voip trust ***

And Avaya S8700 server with the marking change as Voip Barear traffic as dscp : 46(EF) and Voice signaling using dscp : 26 (AF31).

One more thins is that : On Cisco Catalyst 2960 Lan Lite switches there is no option of auto-qos. Hence need to go with manual configuration.

Is it ok if I will configure :

1. cos-dscp mapping accodrding to cos-dscp 0 8 16 24 32 46 48 56

2. All the ports with Ip phone are configured with mls qos trust cos or dscp.

3. Is it ok if I will not configure any thing related to thresold, badwidth and buffer as I can not find all the cmds in this models which i can see in quto generated config in 2960 LAN Base model?

4. What to configure in case of Autoqos and manual configration for Outlook Communicator as video conferecning will be done by OCS client also.

5. Any third Party free tool to monitor the QOS ?

Pls suggest if I m wrong in any case.

Thanks in advance.

Regards,

Shashank Singh · ‎08-16-2011

Hi Dipesh,

Answers inline.

1. All access Ports with switch---IP Phone----PC/Laptop as *** auto-qos voip trust ***.

Yes, this sounds good.

2. All Access ports with Printers,Servers without Voice Traffic with *** auto-qos clasify police ***

Yes, this should be good as long you are OK to let this traffic be policed if need be. This command may remark the traffic so make sure you are aware of this.

3. All the trunk ports on Access, Distribution and CORE switch with *** auto-qos voip trust ***

auto-qos voip trust should be used on ports dirrectly connectd to a voip source. On trunks, mls qos trust cos|dscp should be fine.

4. Port connected to Router with *** auto-qos voip trust ***

This again can be mls qos trust cos|dscp.

Is it ok if I will configure :

1. cos-dscp mapping accodrding to cos-dscp 0 8 16 24 32 46 48 56

This is the default cos-dscp map on the switch. Do not alter this unless need be.

2. All the ports with Ip phone are configured with mls qos trust cos or dscp.

sounds good.

3. Is it ok if I will not configure any thing related to thresold, badwidth and buffer as I can not find all the cmds in this models which i can see in quto generated config in 2960 LAN Base model?

To get started, this is not required. This may be needed once the QoS is in place and anything looks suboptimal.

4. What to configure in case of Autoqos and manual configration for Outlook Communicator as video conferecning will be done by OCS client also.

There is no Auto QoS template specially for Outlook Communicator. You may want to take a look at auto qos video though and check if the generated configuration suits your needs.

5. Any third Party free tool to monitor the QOS ?

Don't take me wrong but with this question, I will just depend on Google

Cheers,

Shashank

sg_network · ‎08-15-2011

Dear Shashank,

I would like to ask the following questions.

1) Is it best practice to turn on (DSCP)-based queue mapping for 10G WS6708 line card? If yes, which scenario we need to turn on DSCP based queue mapping?

2) Does 10G line card required any special tuning for Voice traffic?

3) Is there any performance difference DFC3C and DFC3CXL?

4) Does Cisco phone able to set DSCP instead of COS?

Thanks,

John

Shashank Singh · ‎08-15-2011

Hi John,

Please find the answers inline.

1) Is it best practice to turn on (DSCP)-based queue mapping for 10G WS6708 line card? If yes, which scenario we need to turn on DSCP based queue mapping?

This is not a best practice and not the default behavior. queue-mode by default is mode-cos. You can change the mode to mode-dscp if your traffic is marked with DSCP instead of CoS. With queue-mode as CoS switch uses the cos-dscp map to convert it into an internal dscp before applying QoS. With mode set as mode-dscp, the incoming DSCP is directly used for applying QoS.

2) Does 10G line card required any special tuning for Voice traffic?

Voice traffic is prioritized using the priority queues on switches. Priority queue is serviced until empty and hence traffic going to this queue gets priority over others. If your voice traffic is cos5/dscp 46, switch by default maps it to the priority queue and no spcial tuning needs to be done. However, if your voice is marked with smoething else, you may need to configure the interface to send voice to the priority queue.

3) Is there any performance difference DFC3C and DFC3CXL?

DFC3CXL offers more scalability in terms of routes and NetFlow entries supported compared with DFC3C. There is no difference from QoS point of view that I am aware of. http://www.cisco.com/en/US/solutions/collateral/ns340/ns517/ns224/ns668/net_qanda0900aecd80534905.html

4) Does Cisco phone able to set DSCP instead of COS?

Most of the cisco IP Phones mark the call signalling as dscp 24 and RTP traffic as dscp 46 apart from the CoS marking. If this is not the case with your phone, I would suggest putting this up in the voice forum on the community to seek expert advice.

Hope that helps.

Cheers,

Shashank

sg_network · ‎08-16-2011

Dear Shashank,

Thank you for reply..

for Q1)

If I am not wrong, COS is based on L2 and DSCP is based on L3?

Also COS will loss when L2 medium changed?

In that case, what is best practice for trust COS or DSCP between Access and Distribution?

If we trust DSCP then should used DSCP queuing mode?

for VSL link qos -

If traffic cross the VSL.. does it convert the DSCP to COS then reconvert back at another side?

for 3750 qos stastic -> show mls qos int stastic

Is there any equivalent command at Cat6?

for Cat6 Is there way to see how the qos internal changes?

dscp and internal-dscp are different is same?

Thanks,

John

Shashank Singh · ‎08-16-2011

Hi John,

If am not wrong, COS is based on L2 and DSCP is based on L3?

You are right. CoS is present in the dot1q tag in the ethernet header and is hence layer 2. DSCP is a L3 field.

In that case, what is best practice for trust COS or DSCP between Access and Distribution?

If your traffic has both cos and dscp fields marked, it is advisable to trust dscp on the access/distribution switches.

If we trust DSCP then should used DSCP queuing mode?

Not necessarily. Use DSCP queuing mode if you dont want the switch to calculate an internal DSCP from the fields in the packet.

If the queueing mode is mode-cos (default), and trust state on the interface is dscp, switch will derive an internal dscp from the external dscp which may not be same. Internal dscp will be used by the switch to apply QoS. However, if you change the queueing mode to mode-dscp, the switch will use the dscp marking on the packet instead of deriving an internal dscp.

for VSL link qos - If traffic cross the VSL.. does it convert the DSCP to COS then reconvert back at another side?

On VSL, switch derives euqivalent CoS value from the dscp value (if traffic is untagged) using the cos-dscp map. If there is a CoS marking present, it is simply trusted. Note that this is only from switch's perspective and no change is made to any fileld in the packet traversing the VSL.

for 3750 qos stastic -> show mls qos int stastic Is there any equivalent command at Cat6?

Unfortunately there is no equivalent command on Cat6k. However, 'show mls qos ip' and 'sh queueing interface' provide similar information.

for Cat6 Is there way to see how the qos internal changes?

I am not aware of a show command that tells this on 6k platform.

dscp and internal-dscp are different is same?

Dscp and internal-dscp may not always be same. To understand how internal dscp is derived visit

http://www.cisco.com/en/US/products/hw/switches/ps708/products_qanda_item09186a00804d2e3a.shtml#q44

Cheers,

Shashank

sg_network · ‎08-16-2011

Thanks Shashank,

>> no change is made to any fileld in the packet traversing the VSL.

Thats means can't design Campus qos not more that 8 Class?

If I am not wrong, Base line Qos model is more than 8 Class... How do we achieve with VSL?

>> Unfortunately there is no equivalent command on Cat6k.

>> However, 'show mls qos ip' and 'sh queueing interface' provide similar information.

Is there any other way? How about NetFlow is possible to see stastical?

WS-X6708-10G-3C have RX 8q4t and TX 1p7q4t?

Is there any theories behind for that..

Also.. Is there difference between tail drop and WRED drop?

RX - T is tail drop threshold

TX - T is configurable WRED-drop thresholds or non-onfigurable WRED-drop thresholds

Is possible to combine Interactive Video and Streaming Video together with Cos6?

Thank you,

John

Shashank Singh · ‎08-16-2011

Hi John,

Please find the answers below. I did not get your question regarding configurable / non configurable WRED thresholds. Please elaborate on that one.

>> no change is made to any fileld in the packet traversing the VSL.

Thats means can't design Campus qos not more that 8 Class?

If I am not wrong, Base line Qos model is more than 8 Class... How do we achieve with VSL?

Unfortunately trust CoS is the only option available with VSL which means that all the traffic traversing through the VSL will belong to one of the eight CoS values and will be accordingly queued on the VSL ports. Hence it is recommended to have sufficient bandwidth for the VSL to avoid any congestion.

>> However, 'show mls qos ip' and 'sh queueing interface' provide similar information.

Is there any other way? How about NetFlow is possible to see stastical?

Yes netflow provides granular statistics and lets you analyze data more effectively.

WS-X6708-10G-3C have RX 8q4t and TX 1p7q4t?

Is there any theories behind for that..

These expressions indicate the port level queue architecture of this line card. RX 8q4t means that the port has 8 queues with 4 threshold each on the ingress. 1p7q4t means that the port has 1 priority queue, 7 regular queues and 4 thresholds per queue.

Also.. Is there difference between tail drop and WRED drop?

Tail drop treats all traffic equally and does not differentiate between classes of service. Queues fill during periods of congestion. When the output queue is full and tail drop is in effect, packets are dropped until the congestion is eliminated and the queue is no longer full. In this case the last traffic going into the queue (tail) gets dropped.

On the other hand, WRED makes early detection of congestion possible and provides a means for handling multiple classes of traffic. WRED can selectively discard lower priority traffic when the router begins to experience congestion

Is possible to combine Interactive Video and Streaming Video together with Cos6?

Yes, you just need to classify and mark both traffics to CoS 6 (if they come in with different markings). As long as they are marked as CoS 6, they will be treated in the same way by the switch.

Cheers,

Shashank

sg_network · ‎08-17-2011

Thanks Shashank,

I would like to ask about Cat6 DSCP-COS rewrite at Egress Port

http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/122sxscg.pdf

Figure 41-4 PFC QoS Features and Component Overview

Does it affect the DSCP 46 rewrite to Cos5..

If I am not wrong, cisco default Cos5 to DSCP is not 46.

Thanks,

John

Shashank Singh · ‎08-17-2011

Hi John,

dscp to cos / vice versa conversion is governed by maps. By default dscp 46 maps to cos 5. See below. Left hand column is first digit of DSCP value. Top row is second digit of DSCP value. So DSCP 8 maps to CoS 1 and DSCP 46 maps to CoS value 5.

6509#show mls qos map dscp-cos

Dscp-cos map: (dscp= d1d2)

d1 : d2 0 1 2 3 4 5 6 7 8 9

-------------------------------------

0 : 00 00 00 00 00 00 00 00 01 01

1 : 01 01 01 01 01 01 02 02 02 02

2 : 02 02 02 02 03 03 03 03 03 03

3 : 03 03 04 04 04 04 04 04 04 04

4 : 05 05 05 05 05 05 05 05 06 06

5 : 06 06 06 06 06 06 07 07 07 07

6 : 07 07 07 07

Similarly for cos to dscp conversion, there is a separate map. cos 5 gets converted to dscp 46 as evident from the map below

6509#show mls qos maps cos-dscp

 Cos-dscp map:
        cos:   0  1  2  3  4  5  6  7
     --------------------------------
       dscp:   0  8 16 26 32 46 48 56

Hope that helps,

Shashank

sg_network · ‎08-17-2011

Hi Shashank,

Is there way to disable DSCP-COS rewrite at Cat6? or it is fixed..

If switch convert DSCP - COS rewrite.. what happen to original packet dscp value?

packet ( dscp 46) -> PFC rewrite -> cos 5 (*)

At * stage.. what is dscp value?

I would like to know what is main reason PFC to rewrite at Egress port..

Another things was default cos-dscp map for Cos 5 is dscp 40

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml#concept21

Distribution1#show mls qos maps cos-dscp   Cos-dscp map:
        cos:   0  1  2  3  4  5  6  7
     --------------------------------
       dscp:   0  8 16 24 32 40 48 56

Thanks,

John

Shashank Singh · ‎08-18-2011

Hi John,

DSCP-COS rewrite is actually a method of calculating the COS value from given DSCP. Technically DSCP cannot be rewritten with a COS as they are present in different layer headers.

When switch has to send a packet out on a dot1q trunk interface, only then there may be a need to calculate COS value from the DSCP value. Before dot1q tag is stripped on the ingress trunk interface, switch finds out an equivalent DSCP value and uses it for queueing within the switch. On egress trunk, PFC uses this DSCP to derive the COS and write it back in the dot1q tag so that the marking is retained on the egress.

Cheers,

Shashank

sg_network · ‎08-18-2011

Hi Shashank,

Thanks for reply.

If egress is Layer 3 link. does it still write back COS? or only required for dot1q link.

I heard Cat66 with PFC4 new card? Is it for IPv6 qos upgrade?

Does IPv6 qos for cat6 is same as IPv4?

Also I did not find default cos5 is assign to dscp 46.

below is the show result from VSS.

Do we need to modify?

Cos-dscp map:

cos: 0 1 2 3 4 5 6 7

------------------------------------

dscp: 0 8 16 24 32 40 48 56

Thanks,

John