Cat6500 QoS-Question - DSCP mapping to Queues

[hleschin]

Hello,

we use some Cat6500Sup720Base (Native-IOS 12.2.18SXD4) as router in the core. All interfaces of these devices are real Layer-3 (no VLAN-Interfaces).

Now we have to implement QoS. How is it possible to put received and transmit traffic to different queues (standard and priority) based on the DSCP-value?

We have currently configured:

global mode

mls qos

no mls qos rewrite ip dscp

interfaces

mls qos trust dscp

The modules in the chassis have a strict priority- and another queue (Rx1p1q4t Tx1p2q2t).

In all documentation that I found was no information about the mapping from IP-traffic to receive/transmit queues based on the received DSCP-value. I know how I can to this e.g. with the Cat3750 or Cat3560 platform.

All what I found was:

Mapping CoS Values to Tail-Drop Thresholds

Mapping CoS Values to Strict-Priority Queues

Mapping CoS Values to Standard Transmit-Queue Thresholds

Mapping CoS Values to Standard Receive-Queue Thresholds

I need exactly these steps, but based on DSCP. But because the devices are pure Layer-3, there are no dot1q-trunks and so also none CoS values.

Thanks

H.L.

[bhedlund]

H.L.,

Here is a link to a document that I think you will find useful:

http://www.cisco.com/application/pdf/en/us/guest/netsol/ns432/c649/ccmigration_09186a008049b062.pdf

Additionally I have these comments about your post:

- By disabling qos rewrite you are somewhat defeating the purpose of establishing trusted interfaces. I would suggest leaving qos rewrite enabled and continue defining trusted interface. By default, a trusted interface will not have its qos value remarked to 0 unless it is further defined in a policy to do so.

- On a Sup720 with PFC3, packets transmitting on trusted interface with DSCP values mapping to a COS of 5 will automatically be placed in the strict priortiy queue.

- Receive queue priorities are based on COS, so yes, pure Layer3 intefaces will not be available for special receive queuueing policies.

- You can assigin QoS marked traffic to queues and thresholds on transmit using 'wrr-queue' statements as defined in the attached document.

- Read the 6500 PFC QoS Configuration guide if you have not already.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/122sx/swcg/qos.htm

Hope this helps. Please rate if it does.

Regards,

Brad

[johgill]

"- On a Sup720 with PFC3, packets transmitting on trusted interface with DSCP values mapping to a COS of 5 will automatically be placed in the strict priortiy queue." If the linecard has a priority queue.

[johgill]

The key here is that we queue based on CoS value. Your DSCP->CoS mappings control the queueing of egress packets. From there, you can move CoS wherever you like and that is where the equivalant DSCP value will go. Since you trusted DSCP, you will map from DSCP->CoS for queueing. On the way out, you must make sure your CoS->DSCP maps mirror the DSCP->CoS maps so that you do not change the markings.

I also see you are looking to use "no mls qos rewrite ip dscp" - this is fine, as long as you aren't planning on doing more marking on this switch. If you are strictly trusting and then queueing, you can get away with not worrying about the CoS->DSCP map, but you still need to map DSCP->CoS appropriately according to your queueing requirements.

[hetao]

There is a question for cos and dscp. If we put cos to wrr queue, then just 8 values can be put on queue&threshold, if we can put dscp to wrr queue, we can have more queue&threshold.

My question, does Sup720PFC support dscp to wrr queue&threshold?

[bhedlund]

"My question, does Sup720PFC support dscp to wrr queue&threshold? "

No. The DSCP value is mapped to COS to decided queue and threshold placements. This is more of a factor/limitation of the Catalyst line cards than it is with Sup720.

IOS routers are capable of DSCP based WRED drop thresholds and queue assignments.

I suggest reading the Enterprise QoS Reference Network Design Guide, and PFC QoS Configuration Guide that I link to in my first post for this thread. These are excellent reads that cover all of the details discussed here.

Please rate all helpful posts.

Regards,

Brad

[johgill]

The sup720 and all linecards in the Catalyst 6000 and 6500 family queue based on CoS, not on DSCP. This is the reason for the mappings.