IP precedence default behavior

[beepmeep]

Hi

I've read somewhere that marking certain traffic with IP Precedence 6, would prioritize this traffic. I'm having a hard time figuring out if IP precedence values 6 & 7 (which is reserved for Control Traffic) is by default prioritized or if QoS will have to be configured?

Can anyone provide an answer or a link to where I can find this information?

[Torbjørn · Accepted solution]

IPP/DSCP values are only used in QoS. Without having QoS configured these values won`t affect prioritization.

[beepmeep]

Thanks for confirming my initial thought.

For anyone coming here in the future, I can also add that routing traffic is marked with DiffServ CS6 (or IP precedence 6) by default. So marking this traffic for IP precedence 6 is pointless either way. (checked with pcap on BGP & OSPF).

[Joseph W. Doherty]

"For anyone coming here in the future, I can also add that routing traffic is marked with DiffServ CS6 (or IP precedence 6) by default. So marking this traffic for IP precedence 6 is pointless either way. (checked with pcap on BGP & OSPF)."

Possibly not always true.

Years ago, I recall on 6500s, Cisco documentation recommended, in egress QoS, to set ToS to desired setting even if traffic was known to already have that ToS marking.  Supposedly, on that platform, this was more efficient than allowing the original (same marking) value to flow through the QoS process.

[Joseph W. Doherty]

"Without having QoS configured these values won`t affect prioritization."

BTW, if QoS is configured, prioritization doesn't require using ToS markings nor require any special treatment for specific packet kinds or specific ToS markings.  Such a marking guarantees nothing.

I.e. an active QoS configuration may optionally use a ToS marking however it wants to.

Also, BTW, the whole purpose of the ToS is to provide an efficient way to analyze a packet for servicing purposes.

[Joseph W. Doherty]

Traditionally, by default, Cisco routers or switches just did best effort for all traffic (except if device provided pak_priority [not prioritized, per se, mostly just not egress queue dropped] for some control traffic, which might also be tagged with IPPrec 6).

Some later series Catalyst switches (I recall, starting with 3650/3850) begun having a very basic QoS default, which might vary between device series (QoS features also vary between device series, especially for switches).  So, QoS defaults depends on the device.

BTW, (as you correctly note) per RFC recommendations, "ordinary" traffic shouldn't be tagged with IPPrec 6 or 7, as both are recommended for network operational management.

To your question should you provide explicit QoS for IPPrec 6 or 7 traffic, maybe.  If pak_priority is being used for it, possibly that's good enough, but I'm sure there may be cases where it's not.