QoS, queuing on MPLS link, can I use DSCP based WRED?

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Jun 25th, 2010
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I'm working on a QoS configuration for an MPLS backbone, and have a question on the use of WRED within a policy map. Reading the ciscopress End to End QoS Network design book, it implies that a queuing policy on an MPLS egress link can use WRED to drop traffic based on DSCP preference.

But, I thought the whole point of MPLS QoS was to use the EXP bits in the MPLS header, so the SP QoS policy will have no more than 7 classes, as this is the limit imposed by the EXP bits.

I set up a test in GNS3 to prove/disprove my thoughts using 7200 ipservices code, where I marked telnet traffic with DSCP EF, on a router connected to the CE.

Inbound to the CE I configured a policy matching EF, copying the EF DSCP to EXP 5, MPLS short pipe mode. On the MPLS Egress interface I configured a policy with 2 classes, one matching EXP 5 and the other DSCP EF, the idea being to see if the egress policy can actually look into the IP header past the MPLS header.

I got matches against EXP5, which I expected, no matches against DSCP EF, I wasnt sure about this result. I suppose I need to check if the original DSCP marking is unchanged.

So, this test indicates that if I'm using a 7200 I cannot use WRED on an MPLS encapsulated link, is this the same for other hardware platforms? The eventual solution will be 7600 based, very different to the 7200, I'm just trying to get an understanding of how the MPLS and QoS policies interact and am seeking guidence from anyone who has used this for real.

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Giuseppe Larosa Fri, 06/25/2010 - 03:24
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Hello Aacole,

I would suggest to define traffic classes using the match-any directive in this way you can match on EXP xx OR on DSCP XX, this may take in account unlabelled traffic (if any, depending on the context).

We did so in the past on C7500 and C7200 with good results (we were using LDP label filtering so there was a traffic mix of IP and MPLS packets)

Other platforms like GSR 12000 are able with a single match command to handle MPLS traffic and IP traffic.

I'm not sure about C7600 but an approach with match-any and dual match could be correct.

Hope to help


chintan-shah Sun, 06/27/2010 - 22:59
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Hello Aacole,

I'm not getting what you are trying to achive but in MPLS egressl link towards P, You should be alwasy doing Qos/WRED based on MPLS EXP.

I would suggest you match your dscp at ingress to CPE , Set  Qos-group and discard-class along with MPLS EXP at egress to P, you can match Qos group in class and do Qos and on WRED can be done based on discard-class that's what Most of SP does in their network for MPLS based Qos.

If you want to do short pipe mode, on egress to CPE at remote end you can alwasy match dscp and do Qos but I would suggest for PIPE MODE again so that you have consisent policy and also you manager you network Qos based on you policy....Please note 7600 don't support PIPE MODE and only support Short pipe mode.



aacole Thu, 07/01/2010 - 05:38
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Hi Chintan

I was trying to understand if I can use WRED based on DSCP on an MPLS encapsulated link. This was suggested in my Carrier Ethernet course notes based on 7600, but I wasn’t sure if it was correct. I recall that the only QoS markers I can use on an MPLS link are EXP or QoS group.

You talk about using WRED based on EXP bits, I will read up on this. I'm ok with the QoS on the Ethernet links, I'm not so sure about my options on the MPLS links.

chintan-shah Mon, 07/05/2010 - 00:17
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Hi ,

Please bear in mind that 7600 doesn't support PIPE MODE hence you have no option to do Qos based on Qos-group or EXP.

You have no choice but have to do  WRED based on dscp/precednace only.




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