lee.reade Mon, 06/30/2008 - 07:28
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Do you mean cos and tos?

Well cos is a tag found in 802.1q and ISL encasulated frames ie layer 2, it has 8 possible values available, 0-7, for setting cos. Note that it is only found between two devices configured as a trunk, so once the frame is layer 3 forwarded, the 802.1q or ISL header is removed.

TOS, is the original IP specification for type of service and is layer 3 based, ie IP. You can use either ip precedence which has 8 values available, again 0-7, for setting the precendence, however the TOS field was increased from 3 bits to 6 to form the new dscp fields, so you know can have 64 possible values for IP dscp qos settings.

You would use cos when doing switch to switch traffic at layer 2, whereas you would use ip precedence or dscp when doing layer 3 routing etc, ie say you wanted to mark all traffic from one site going to another and provide that site to site traffic x amount of bandwidth, you would mark the dscp bits with a value and set you qos policy to provide the x amount of bandwidth.

Anyway, thats it in a nutshell.

Have a look at ;




carl_townshend Tue, 07/01/2008 - 00:38
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thanks for that


Why do switches have cos to dscp maps? and do routers have the same? If I mark the cos value at my layer 2 switch level, and this hits my router with the cos value of say 6, what and how will my router map this and mark the dscp field ?

lee.reade Tue, 07/01/2008 - 00:50
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Switches have cos-dscp maps, becaused whenever a packet gets layer 3 forwarded all the previous information in the ISL or 802.1q header is gone, so it uses the maps to copy the cos setting to dscp, as per the map, before stripping the ISL or 802.1q header off, otherwise, it just gets set a 0.

Well it depends if your router is configured to do any qos actions, qos is something that you define, you need to design your qos policy and understand what gets marked where etc.

Its kinda hard to explain it all, as qos is a pretty big subject, sorry.



ashok_boin Tue, 07/01/2008 - 02:11
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To answer your question straight...

Remember, it's local COS-DSCP map which will be used. So, if you are talking about at router, it will use it's local map for eg if you have configured to map CoS 6 to DSCP 48, then it will do it accordingly.

As CoS value is the property of dot1q trunk configurations, it's usual that it will be just DSCP (assuming no trunk b/w core switch & router) by the time it reaches your gateway.



carl_townshend Tue, 07/01/2008 - 05:42
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hi there

thanks for the replies. So if I apply cos level 6 to a specific vlan on my switch, does it tag that traffic going out of the trunk port with cos value of 6? does it tag the dscp value in the ip header also??? or when it reaches my core multilayer switch trusted port does the multilayer switch then mark the dscp value?

please help ?

Collin Clark Tue, 07/01/2008 - 06:16
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You tag specific traffic, such as voice bearer, not all vlan traffic. The CoS is applied either at the device, like an IP phone, or at the trust boundary, which is usually the access switch port. The switch/device may or may not tag DSCP values, depends on the device and the switch configuration. Routers do not recognize CoS values, so at some point a CoS to DSCP mapping will occur and that should happen at the sending device or at the trust boundary. If you have legacy switches that do not recognize DSCP, you would then have to configure DSCP-COS maps on the router. This would be beyond your WAN link so it may not be necessary.

Hope that helps.

carl_townshend Tue, 07/01/2008 - 07:47
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hi there, so if my switch has a cos to dscp map in it, will the switch add the dscp value to the packet according to the cos tp dscp map?

Collin Clark Tue, 07/01/2008 - 07:52
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Yes, that's the reasoning of the map.


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