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QoS and DSCP - Question

chriwall01
Level 1
Level 1

Hi all,

I was wondering if someone could clarify something for me... I'm teaching myself QoS and have been playing!  Basically I set up a lab with the following

SWITCH #1 -- ROUTER #1 -- ROUTER #2 -- SWITCH #2

On SWITCH #1 I have enabled QoS globally with the 'msl qos' command.  I have also created an access list to match any TCP traffic on port 5001 (default iperf traffic), and created the class-map and service policy to say, any packets that match this ACL set the DSCP to 'EF'.  This service policy is assigned to the ingress of an interface on SWITCH #1... GREAT!

I can test this by plugging in a laptop into the interface with the service-policy applied on SWITCH #1 and send iperf traffic to another laptop plugged into another interface again on SWITCH #1.  When running Wireshark on ther recieving computer at the same time I can see this traffic with port number of 5001 having Expidential Forwarding (EF) DSCP values!!!

However I then plugged the recieivng laptop into SWITCH #2 and ran the same tests.  When I looked at the Wireshark results I saw that the traffic was still being marked with the correct DSCP value.  Now I'm probably getting this all wrong, but I was expecting to see it with the DSCP value stripped off as NO interfaces other than the one the transmitting laptop is plugged into has the 'mls qos trust dscp' command assigned.  I was always under the impression that you had to have 'mls qos trust dscp' assigned on evey interface the traffic passes through on any device so that the DSCP can be kept??  Obviously I am wrong, so was wondering if someone would be kind enough to explain?  Is it the case that because the DSCP is in the L3 header it will just go through, or is it because I haven't enabled QoS on the other device it is just carrying it through (sorry just thought of that and I'm not near my lab to test it out!)

Any help would be great, hope i've given enough detail - but if not then please let me know.

Ta

1 Accepted Solution

Accepted Solutions

Hi,

or is it because I haven't enabled QoS on the other device it is just carrying it through

You answered your question. If you don't configure anything then it doesn't care about the tags and so they are left untouched.

Regards.

Alain.

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View solution in original post

4 Replies 4

Hi,

I hope it use the normal queuing in MLS not enabled interface.. If there is any congestion then the interface will use the normal queuing only..

I think i am right.. Any one need ur advice also..

Thanks,

Jayaram..

Hi,

or is it because I haven't enabled QoS on the other device it is just carrying it through

You answered your question. If you don't configure anything then it doesn't care about the tags and so they are left untouched.

Regards.

Alain.

Don't forget to rate helpful posts.

Hi Alain,

Thanks very much, for replying - now I just need to get my head round the queueing aspects of QoS.

Thanks!

Hi,

Cisco software routers (e.g. 28xx, 38xx, 72xx), usually don't modify the L3 ToS byte in the IP Header unless you configure them to do so.

However, if you had a switch with mls qos enabled globally in the middle then by default the ports become untrusted and the DSCP values dont get honored and get re-marked to 0

HTH,

Regards,

Kishore

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