DSCP Markings and non-QoS IOS

[Keith Joel]

Hi All,

I'm dealing with a customer that has an end of life router with a pre-QoS IOS image.  In an egress scenario, the router lies before their MPLS WAN router which is where any of the shaping happens.  We are looking to implement QoS across the WAN link for voice but I'm not sure whether this old equipment will ignore the DSCP values and pass them along or whether it will re-write the ToS field of the IP header.  We are not looking to do any shaping or classification on this device but we need it to pass these values to the MPLS router.  Has anybody encountered this scenario?

Thanks for your time,

Keith

[Karsten Iwen · Accepted solution]

If there is no QoS configured, your router will just ignore the markings and will also not change them.

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[Alessio Andreoli]

Hi,
If the IOS is not QoS capable(which version is it?) then everything will be forwarded not clearing about any dscp value. From your perspective is like if a QoS would be in place with only class default configured on the policy. Whatever you need to do with VoIP will require an IOS upgrade.

Alessio

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[Joseph W. Doherty]

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Posting

As the other posters have already described, Cisco routers will pass along ToS unless configured otherwise.

Cisco routers, if running original interface WFQ, will treat packets differently based on ToS markings. (Interface WFQ was default for E1 and slower interface.  Other older QoS configurations can also use ToS markings.)

Unless you're working with a really, really old Cisco router, pre-DSCP ToS can be worked with using the older IP Precedence (and optional DTR bit settings - this needed if you need to work with DSCP EF).

[Keith Joel]

Thank you everybody for your responses.  I realized after I posted this that we had a 3640 router in our lab so for "fun" I mocked up the scenario.  As the first response idicates, the router ignored the DSCP values and simply passed them on which is what I wanted in the least.  The important thing was that the WAN router behind it received those values.  It's not true end to end QoS with the 3640 treating everything alike but until they upgrade it'll have to do.

[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

... with the 3640 treating everything alike ...

Although ToS values, by default, are usually passed through a router unchanged, again, depending on the interface and its default configuration, every packet might not be treated alike if WFQ is active.