DSCP Question

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May 11th, 2009
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I have reached the DSCP section of ONT, so far i think I'm understanding it fairly well...


The only thing i don't quite understand yet (and maybe i just haven't reached this section yet) is how or what defines the type of traffic that belongs to a certain DSCP value.


so i can define a class-map saying 'match dscp af43' but what traffic as af43 referring to?

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Joseph W. Doherty Mon, 05/11/2009 - 02:47
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The application that sources the traffic can mark with the DSCP it desires to use. Network devices that "understand" DSCP can set or reset DSCP based on criterial it chooses. There are both DSCP marking and DSCP processing recommendations, but for any one instance, actual meaning and/or processing often requires knowledge of what's happening within your network.


[edit]

For example, traffic marked with DSCP EF most likely is VoIP related and given best priority treatment, but it could actually be some backup application traffic and/or given no or worse possible treatment. (No treatment isn't unusual, worse possible treatment would be unsual since it's so contrary to recommendations.)

LucaSalvatore_2 Mon, 05/11/2009 - 02:51
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Ok so something like an IP Phone could mark voice traffic with EF or something similar then?


are the commands on a router or switch to say something like, put http, ftp and ssh into af11 for example?

Joseph W. Doherty Mon, 05/11/2009 - 02:57
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"Ok so something like an IP Phone could mark voice traffic with EF or something similar then? "


Yes, most IP phones would mark their voice payload packets with DSCP EF, other of their packets, e.g. signally, ping responses, with other DSCP markings.


"are the commands on a router or switch to say something like, put http, ftp and ssh into af11 for example?"


By default, normally no. By manual configuration, often possible, routers especially. Switches, if just L2, might not support DSCP, if L3 often some support for DSCP.


[edit - to clarify last answer, QoS commands supported depend on features supported by device, but most device QoS features, by default, are often disabled.]


nate-miller Mon, 05/11/2009 - 13:52
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It's possible to tag every packet coming out of your Windows workstation as EF, if you care to dig around in the NIC drivers. THis is why it's a bad idea to blindly trust end-user ports!


And yes, in a Cisco l3 switch, you can write policy maps that identify traffic based on any access list you write, and then re-mark the traffic so it falls into any DSCP marking you'd like.


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