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New Member

question in number of makrings when using dscp

hi  all

i want to ask about dscp

how many markfings it can make whgen using it ???

as i understood

i have

ef

af1-4

cs1-7

which in total about 18

but the question is whhy there is 64  value can be put ???

i may have some missunderstood

wish to clarify me

==============

<0-63>   Differentiated services codepoint value

af11     Match packets with AF11 dscp (001010)

  af12     Match packets with AF12 dscp (001100)

  af13     Match packets with AF13 dscp (001110)

  af21     Match packets with AF21 dscp (010010)

  af22     Match packets with AF22 dscp (010100)

  af23     Match packets with AF23 dscp (010110)

  af31     Match packets with AF31 dscp (011010)

  af32     Match packets with AF32 dscp (011100)

  af33     Match packets with AF33 dscp (011110)

  af41     Match packets with AF41 dscp (100010)

  af42     Match packets with AF42 dscp (100100)

  af43     Match packets with AF43 dscp (100110)

  cs1      Match packets with CS1(precedence 1) dscp (001000)

  cs2      Match packets with CS2(precedence 2) dscp (010000)

  cs3      Match packets with CS3(precedence 3) dscp (011000)

  cs4      Match packets with CS4(precedence 4) dscp (100000)

  cs5      Match packets with CS5(precedence 5) dscp (101000)

  cs6      Match packets with CS6(precedence 6) dscp (110000)

  cs7      Match packets with CS7(precedence 7) dscp (111000)

  default  Match packets with default dscp (000000)

  ef       Match packets with EF dscp (101110)

=============================

regards

1 ACCEPTED SOLUTION

Accepted Solutions

Re: question in number of makrings when using dscp

Hi,

I think this is much easier to understand when we take a look at how the ToS-Field of the IPv4 header has changed over the years.

Originally (RFC 791, 1349), we had only 3 bit which where used for the so-called IP-Precedence (IPP), pretty much like the CoS-field in 802.1Q (layer-2 marking).

So we had 8 possible markings (classes) from 000 to 111 (0 - 7).

With RFC 2474, 2 bit were added to define different "Drop Probabilities" within a main class, so now we could define which packets of a certain class (or queue) will be dropped first when it comes to congestion. This gives much more granularity in the treatment of different classes.

Actually the DSCP field now had 6 bits, but the last one has to be set to 0, so we have 2^5 (=32) possible values.

The class selectors are for IPP backwards-compatibility, the drop probability is set to 0, so the markings are the same like IPP markings:

000|00|0: BE (Best Effort)

001|00|0: CS1=IPP1

010|00|0: CS2=IPP2

011|00|0: CS3=IPP3

(...)

However, like mentioned above, with DSCP we have two more bits for the drop probability. They are used in the Assured Forwarding (AF) Groups 1-4:

000|00|0: BE (Best Effort)

001|00|0: CS1=IPP1 (decimal: 8)

001|01|0: AF11 (10)

001|10|0: AF12 (12)

001|11|0: AF13 (14)

010|00|0: CS2=IPP2 (16)

010|01|0: AF21 (18)

010|10|0: AF22 (20)

010|11|0: AF23 (22)

011|00|0: CS3=IPP3 (24)

(...)

CS7 is reserved, CS6 is for network control traffic (like routing-protocol traffic), so an additional drop probability wouldn't make too much sense here.

In group 5 we have only CS5 (101|00|0 / 40) and EF (Expedited Forwarding, 101|11|0 / 46), so we have 20 usable values which is more than enough.

HTH

Rolf

4 REPLIES

Re: question in number of makrings when using dscp

Hi,

I think this is much easier to understand when we take a look at how the ToS-Field of the IPv4 header has changed over the years.

Originally (RFC 791, 1349), we had only 3 bit which where used for the so-called IP-Precedence (IPP), pretty much like the CoS-field in 802.1Q (layer-2 marking).

So we had 8 possible markings (classes) from 000 to 111 (0 - 7).

With RFC 2474, 2 bit were added to define different "Drop Probabilities" within a main class, so now we could define which packets of a certain class (or queue) will be dropped first when it comes to congestion. This gives much more granularity in the treatment of different classes.

Actually the DSCP field now had 6 bits, but the last one has to be set to 0, so we have 2^5 (=32) possible values.

The class selectors are for IPP backwards-compatibility, the drop probability is set to 0, so the markings are the same like IPP markings:

000|00|0: BE (Best Effort)

001|00|0: CS1=IPP1

010|00|0: CS2=IPP2

011|00|0: CS3=IPP3

(...)

However, like mentioned above, with DSCP we have two more bits for the drop probability. They are used in the Assured Forwarding (AF) Groups 1-4:

000|00|0: BE (Best Effort)

001|00|0: CS1=IPP1 (decimal: 8)

001|01|0: AF11 (10)

001|10|0: AF12 (12)

001|11|0: AF13 (14)

010|00|0: CS2=IPP2 (16)

010|01|0: AF21 (18)

010|10|0: AF22 (20)

010|11|0: AF23 (22)

011|00|0: CS3=IPP3 (24)

(...)

CS7 is reserved, CS6 is for network control traffic (like routing-protocol traffic), so an additional drop probability wouldn't make too much sense here.

In group 5 we have only CS5 (101|00|0 / 40) and EF (Expedited Forwarding, 101|11|0 / 46), so we have 20 usable values which is more than enough.

HTH

Rolf

Re: question in number of makrings when using dscp

Here you can find a very detailed document:

Enterprise Medianet Quality of Service Design 4.0

New Member

Re: question in number of makrings when using dscp

fischer.rolf wrote:

Hi,

I think this is much easier to understand when we take a look at how the ToS-Field of the IPv4 header has changed over the years.

Originally (RFC 791, 1349), we had only 3 bit which where used for the so-called IP-Precedence (IPP), pretty much like the CoS-field in 802.1Q (layer-2 marking).

So we had 8 possible markings (classes) from 000 to 111 (0 - 7).

With RFC 2474, 2 bit were added to define different "Drop Probabilities" within a main class, so now we could define which packets of a certain class (or queue) will be dropped first when it comes to congestion. This gives much more granularity in the treatment of different classes.

Actually the DSCP field now had 6 bits, but the last one has to be set to 0, so we have 2^5 (=32) possible values.

The class selectors are for IPP backwards-compatibility, the drop probability is set to 0, so the markings are the same like IPP markings:

000|00|0: BE (Best Effort)

001|00|0: CS1=IPP1

010|00|0: CS2=IPP2

011|00|0: CS3=IPP3

(...)

However, like mentioned above, with DSCP we have two more bits for the drop probability. They are used in the Assured Forwarding (AF) Groups 1-4:

000|00|0: BE (Best Effort)

001|00|0: CS1=IPP1 (decimal: 8)

001|01|0: AF11 (10)

001|10|0: AF12 (12)

001|11|0: AF13 (14)

010|00|0: CS2=IPP2 (16)

010|01|0: AF21 (18)

010|10|0: AF22 (20)

010|11|0: AF23 (22)

011|00|0: CS3=IPP3 (24)

(...)

CS7 is reserved, CS6 is for network control traffic (like routing-protocol traffic), so an additional drop probability wouldn't make too much sense here.

In group 5 we have only CS5 (101|00|0 / 40) and EF (Expedited Forwarding, 101|11|0 / 46), so we have 20 usable values which is more than enough.

HTH

Rolf

1st of all ,

let me thank you very much for the summary and explanation  you mentioned above ,

so u agree with me about wt i mentioned above , about 20  dscp usable vlaues that  can  be used ,

but the question is , why cisco give us the choice to choose from 0-64 ????

regards

Re: question in number of makrings when using dscp

but the question is , why cisco give us the choice to choose from 0-63 ????

This is actually an interesting question - I've never thought about that.

So I've also learned something new today: In addition to the standard code points (with the least significant bit set to zero), this bit may also be set to 1 for local, experimental or future use:

http://www.iana.org/assignments/dscp-registry/dscp-registry.xhtml

Although rarely necessary, IOS allowes you to configure such values by decimal numbers.

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