qos

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Jan 28th, 2008
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Hi all , when routing with qos over switches then routers, what happens when it crosses over the layer 2 to the next router, as the router will mark the dscp, but then it has to go over my layer 2 infrastructure then to the next hop router.

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royalblues Mon, 01/28/2008 - 04:37
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Most of the new switches now can read the DSCP bits and hence if proper trusting is configured, the DSCP value should be intact


In case the switches do not support DSCP, you can use the DSCP to Cos Mapping


Narayan

Joseph W. Doherty Mon, 01/28/2008 - 05:06
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I believe the L3 ToS should be preserved on a L2 device.


PS:


Some L3 devices have the capability to "map" L3 ToS to L2 CoS and/or L2 CoS to L3 ToS.


So, depending on configuration, a downstream L3 device could remark the ToS based on preferring a CoS over the original ToS.

carl_townshend Mon, 01/28/2008 - 08:52
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thanks for that


so if there is nothing configured on the switches between 2 routers, it should preserve the dscp value ?

Joseph W. Doherty Mon, 01/28/2008 - 18:38
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"Yes", is my understanding. The L3 packet, and its ToS DSCP value, should be left alone within the L2 frame.

senthil_kumarpv Mon, 01/28/2008 - 23:23
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Joseph


Yes it is correct , but in some switches i am seeing the Mapping of DSCP value in L2 switches , if L2 preserve the the value of TOS but why the explicit configurations needed ?


Senthil

Jon Marshall Mon, 01/28/2008 - 23:34
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Senthil


Not sure what you mean but all Catalyst switches use an internal DSCP value for QOS decisions. This internal DSCP value is not actually written into the packet it is just derived from the packet.


Depending on what the port has been set to you may need to use one of the DSCP maps ie.


IPP to DSCP (IPP = IP precedence)

Cos to DSCP


So if you are trusting CoS or IPP on the port the switch needs to derive an internal DSCP value for the packet and that is what the maps are used for.


Jon


Joseph W. Doherty Tue, 01/29/2008 - 04:36
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If you have a pure L2 switch that maps L3 ToS into a L2 CoS, believe you're looking at a "feature". The purpose being to provide a L2 CoS level of service without any CoS value in the original frame but based on the L3 ToS value. I think I recall some switches, not necessarily Cisco, might even have the capability to write a new L2 CoS also based on a L3 ToS.

Jon Marshall Tue, 01/29/2008 - 04:56
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Joseph


I'm a bit confused now. When you talk about a L2 switch mapping L3 ToS to L2 CoS are you talking about when the packet is scheduled to be transmitted rather than when it is received.


It's my understanding that all Cisco switches use an internal DSCP value so why would it need to map L3 ToS to L2 CoS on ingress ?


Jon

carl_townshend Fri, 02/08/2008 - 00:48
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did switches only used to use the cos value?


do most new switches now read the dscp value? but can they only write the 802.1p value?

Joseph W. Doherty Fri, 02/08/2008 - 04:24
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Carl, sorry for the late reply. I only now noticed your question.


Remember CoS is L2 (frame), ToS L3 (packet). A pure L2 switch is less likely to be able to look into or modify the L3 packet, but as Narayan's post shows, a fully featured one may.


L3 switches route packets, so its more like they can work with ToS but that's not really guaranteed, I believe, either.


You also might work with equipment that predates the later CoS and ToS standards.

Joseph W. Doherty Fri, 02/08/2008 - 04:06
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Jon, sorry for the late reply. I only now noticed your question to me.


Yes, when I refer to L3 ToS to L2 Cos, I am thinking of what's in the transmitted frame. Within Narayan's posted reference, that would be such as described in the "Configuring the DSCP-to-CoS Map" section. Reading that section, it's not clear whether the transmitted frame actually has the mapped CoS or whether it's just used for egress queue selection. I would expect the former so that downstream L2 switches can also perform QoS using CoS.


Unsure about all Cisco switches using DSCP values. Likely on advanced L3 switches, not so much on pure L2 switches. Also, original question didn't specifically reference Cisco, so my answers have been generic.


As to where a mapping, if supported, of ToS to Cos is done, agree it may not be necessary on ingress. Would depend on, as you ask, what the switches uses for making internal QoS decisions.

Joseph W. Doherty Fri, 02/08/2008 - 04:15
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Senthil, sorry for the late reply. I only now noticed your question to me.


Why the ability to remap ToS or CoS, depends on what model of QoS you're using. Some (all?) models don't guarantee the ToS or CoS will remain the same end-to-end. There are reason's that any hop might wish to mark up or down. One common reason to mark down, offered bandwidth rate is beyond agreed rate. Non-conforming traffic is marked down.


If you wonder about CoS to/from ToS mappings, could also be you want to take advantage of equipment capabilities but original marking doesn't imply. E.g. A VoIP phone that marks only a DSCP EF. An inline device sees it, and also marks it with equivalent CoS value.

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