IP phone to 3850 to 4500X to CE router - DSCP to COS marking

[roger perkin]

I am implementing a QoS solution for the above topology.

I need to trust the marking from the phone (which I believe is trusted by default on the 3850 and then pass the markings to the 4500X.

The CE router will prioritise traffic with CoS 5 across the WAN

My questions are where do I re-mark the traffic from EF to CoS 5? Should this be on the 3850?

What needs to be done on the 4500X to ensure the markings remain. 

Thanks

Roger

[Leo Laohoo]

My questions are where do I re-mark the traffic from EF to CoS 5? Should this be on the 3850?

Yes.

What needs to be done on the 4500X to ensure the markings remain. 

None.

[roger perkin]

Thank you Peter, 

This is from the ISP

The important thing to note is that this is a Layer2 CoS scheme so a L3 CoS schema can be mapped onto our CoS at the CE router.

So I need to speak to them tmw

But is this saying that we should leave the DSCP intact marking at EF and then get them to re-map EF to an 802.1p value of 5 or maybe they do that already?

[Joseph W. Doherty]

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Posting

Tagged frames only exist until the next L3 hop.  If you were doing (same) tagged L2 end-to-end, the phone's (tagged frame) CoS 5 should be all you need (assuming all transit devices leave it alone).  Otherwise you'll need to regenerate the CoS tag when a new tagged frame is built.  L3 ToS, which can go end-to-end, can be used to know what CoS tag needs to be generated when building such tagged frames.

Assuming you're tagged L2 up to your CE router, you'll need to build a new L2 frame for your "WAN", so tagging that frame correctly would depend on your CE router.

If your phone isn't using tagged frames, but is using L3 ToS, then, as noted above, your CE router can use the L3 ToS marking to chose what L2 CoS tagging needs to be set.  (NB: ideally, packet will preserve the L3 ToS.)

[roger perkin]

Thank you Joseph, 

So if the phone is tagging at L3 (DSCP 46) and the 3850 is trusting the wired ports by default will this marking get to to the 4500X?

Then the 4500X will pass it on. 

I then need to arrange for the ISP router to map DSP46 to L2 (Cos 5)

I see some configs are matching Cos 5 at the switch level. 

I just need to get all these concepts straight in my head. 

Thanks

Roger

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yes, I would expect what you describe to be what you need.

Regarding matching CoS 5 at the switch level, older IOS versions for switches a) generally required trusting of ingress markings, if QoS was enabled, and b) true "L2" switches didn't "see" L3 ToS.

Routers and newer switch IOS versions tend to operate alike, i.e. they only change L3 ToS is so configured.  As Peter mentioned, ideally, on modern equipment you use L3 ToS but if something needs L2 CoS, then you set it where needed.  This might be the case for your WAN egress.

[Peter Paluch]

Roger,

This may actually be avoiding your original question but as far as I can recommend, you should focus on keeping your DSCP marking intact and set to EF, because these markings are the only ones that will survive over the entire end-to-end path. CoS markings will be retained only on trunks, and they will not be carried through routers at all unless the egress interface uses VLANs and you force the CoS marking onto the frames using an explicit policy-map. It does not make much sense to focus on CoS markings, anyway, as both 3850 and 4500-X can take DSCP markings into account and provide differentiated services based on DSCP markings.

Does this sound applicable to your situation?

Best regards,
Peter