More questions on IP Prec and DSCP

[John Blakley]

All,

I understand that there are 6 (0-5) ip prec codes that can be used (6-7 are reserved, but *could* be used), and there are 64 different values for dscp.

In which cases should someone choose one over the other? Is it more preference than anything?

Thanks,

John

[Edison Ortiz]

You mean choosing IP Precedence over DSCP?

Well, DSCP allows you to have more values available for QoS markings.

Say, you have 5 different applications and you want to apply 5 different markings. With IP Precedence, you will have an issue.

HTH,

__

Edison

[John Blakley]

Edison,

But overall, they do the same thing? We just have the option of marking more classes with DSCP?

Thanks,

John

[Edison Ortiz · Accepted solution]

Yes

__

Edison.

[John Blakley]

Thanks

[Jon Marshall]

John

Just one additional point. The AF classes support drop probabilities which are not supported with standard IP Prec values.

This may well be another reason to deploy DSCP.

Jon

[Joseph W. Doherty]

"In which cases should someone choose one over the other? Is it more preference than anything? "

I would recommend attempting to keep within the latest RFCs ([edit] especially RFC 4594), however choice can be dictated by equipment capabilies. Some equipment doesn't support IP ToS, some supports (IP Precedence) RFC 791 or RFC 1349, some RFC 2474 (and later). (I've been able to generate a DSCP value of choice on Cisco equipment that doesn't "know" of DSCP but does "know" of RFC 1349.)

Also be aware that of some Cisco equipment "interprets" ToS beyond your explicit control (e.g. WFQ IP Precedence weighting) or allows you full control (e.g. CBWFQ, BE can be given precedence over EF - that can conflict with the RFCs.)

"But overall, they do the same thing? We just have the option of marking more classes with DSCP? "

Not exactly, although RFC 2474 uses Class Selector Codepoints with rules to preserve backward compatibility with the IP Precedence portion of the ToS, bits 0..2. The rest of the ToS usage is quite different between the RFCs, and RFCs note this, but they also note it's rare to find systems using the rest of the ToS as defined by RFC 791 or RFC 1394. (NB: BTW, some Cisco platforms support using the ToS older standards beyond IP Precedence.)

[edit]

An example of a direct conflict in usage, RFC 3662 suggests CS1 to indicate "A Lower Effort Per-Domain Behavior (PDB) for Differentiated Services", yet Cisco WFQ would treat such traffic better than BE. (The RFC notes there's a conflict in usage, but also the whole point of QoS is to provide the service we desire.)