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Community Member

Diffserv PHB questions

Hello community,

I am a little confused on all the PHB's that the diffserv model of QoS implements. From my understanding they are like this:

Class Selectors: Basically Layer 3 DSCP values in the range of the legacy IPP fields. If a network using the IPP values wants to use diffserv then they will use the CS PHB?

Assured Forwarding: Having a hard time understanding how the switch or router classifies traffic into one of the 4 different classes in this category. What does it have to do with CoS or DSCP?

Expedited Forwarding: This is basically DSCP 46 Voice traffic and only voice traffic? If this is the case its impossible that it gets dropped unless there is severe congestion or if the priority queue is oversubscribed and the voice traffic is non conforming?

CoS values: Layer 2 QoS values placed in the 802.1Q tag. Normally used with an MVAP port connected to an IP Phone.

DSCP values: Layer 3 QoS values in the L3 header of the TOS field. But how does a L2 switch read these values if you set the switchport to trust DSCP values? Also, it is said to only trust these values on an access port with no phone connected. What types of end devices would mark its traffic with L3 DSCP values?

I know this is lengthy but im so confused. Thanks.

Chris.

3 REPLIES

Diffserv PHB questions

Hello.

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Class Selectors: Basically Layer 3 DSCP values in the range of the legacy IPP fields. If a network using the IPP values wants to use diffserv then they will use the CS PHB?

IPP is 3 bits long.

DSCP is 6 bits long; overlapping with IPP by 3 top-most bits.

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Assured Forwarding: Having a hard time understanding how the switch or router classifies traffic into one of the 4 different classes in this category. What does it have to do with CoS or DSCP?

Not sure what are you talking about here. Swiches (like 2960, not 2960S) classify into 2 queues inbound and 4 outbound. Inbound classification depend on qos trust - either CoS or DSCP. By default inbound CS5 goes to queue 2; outbound goes to queue 1. Switch could rewrite CoS and DSCP values.

Routers classify only per service-policy (that is user-defined).

Expedited Forwarding: This is basically DSCP 46 Voice traffic and only voice traffic? If this is the case its impossible that it gets dropped unless there is severe congestion or if the priority queue is oversubscribed and the voice traffic is non conforming?

Usually DSCP ef (equal to 46) or CS5 or CoS 5 is for voice traffic only.

Switches should forward it first.

Router could warrantee minimum and maximum bandwidth using LLQ.

DSCP values: Layer 3 QoS values in the L3 header of the TOS field. But how does a L2 switch read these values if you set the switchport to trust DSCP values? Also, it is said to only trust these values on an access port with no phone connected. What types of end devices would mark its traffic with L3 DSCP values?

Switch is aware of L3 and L4 protocols, so it's not a problem to look into DSCP value and even rewrite it

Any device could mark traffic with DSCP value. It's usually a router or Video device.

It could be an end-station (Windows XP/7), but I won't trust unless I've configured it myself.

Community Member

Diffserv PHB questions

Mik,

Thanks for all your clarification. I guess I am just still confused on how certain traffic gets classifies. I know that Cisco IP phones classify its own voice traffic with EF (DSCP 46 and CoS 5) but how would traffic be classified in the AF category? Are there appliances out there that classify its own traffic with certain QoS markings in the AF category? Thanks.

Re: Diffserv PHB questions

Hello.

Voice end-points mark traffic per VoIP system configuration. And EF/Cos 5 is a standard for voice.

Voice signaling traffic could be classified by voice end-points as well. Best practice is mark it with CS3 (CoS 3).

Routers are marking their traffic:

  • SSH as an option;
  • routing protocols - CS6.

Whenever you design your enterprise QoS policy/model, you would probably mention following classes:

  • Video (end-points mark traffic themselves) - CS4; some could put video into CS5;
  • Interactive critical business applications - CS3 (including voice signaling);
  • Generic business application - CS2;
  • Best effort (no marking);
  • Scavenger - CS1;
  • service traffic CS6.

Typically application marking will be done by switches (mls qos trust or inbound service policy) or routers if you use QoS for WAN congession management.

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