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ONT certification exam..... need understanding IP Precedence...

Level 1
Level 1

Hi !

I'm currently preparing my ONT certification Exam, also for my knowledge I would like to understand some things about IP precedence...

1- someone told my IP Precedence is not really a QoS algorithm. Why exactly ?

2- I understand the only 3 first bits are used for IP Precedence to mark the packet, what are the usage of the reset of this byte ?

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It looks like your book is correct and I am wrong. Which makes the value of 46 for EF make little sense.

View solution in original post

6 Replies 6

Some people consider QoS to be strictly a Layer 3 function. IP Precedence operates only in Layer 2. I think the reasoning for this is that traditionally, the Layer 2 switching function has happened at the hardware ASIC level, whereas 'QoS' is a higher-level function that requires the central processor. Current multilayer switches blur that line to pointlessness and it is now just a distinction without difference.

The first 3 bits represent the IP Precendence field and are the only bits of the field read by a layer 2 device. the next 3 bits, combined with the first 3 bits make up the 6-bit DSCP field. This is why CS1 is often associated with AF11, AF12, and AF13 and CS2 is associated with AF21, AF22, and AF23, etc; Because the first three bits of AF11 are the same as CS1, so a layer 2 device will treat them the same.

The last 2 bits of the ToS byte at the Explicit Congestion Notification and usually have absolutely nothing to do with the packet it is attached to. It is a method for routers to communicate with other routers upstream and down.

Hi !

if I understand you the rest 3 others bits are simply not used or used for something else than QoS if my device is IP Precedece only capable, and the last 2 bits of the same are used for congestion notification is it correct ?

Of the 8-bit ToS field...

bits 0-2 = the 3-bit IP Precedence field, which equate to...

000 = CS0 = Best Effort

001 = CS1

010 = CS2

011 = CS3 (Often used for VoIP signaling)

100 = CS4

101 = CS5 (Often used for VoIP RTP)

110 = CS6 = System

111 = CS7 = Critical

bits 0-5 = the Differentiated Services Control Point

This is compatible with the IP Precedence bits, in that CS1, CS2, CS3 and CS4 map to AF1x, AF2x, AF3x, and AF4x. Bits 3 and 4 make up the drop probability and divide the AF (Assured Forwarding) classes in AFx1 (high drop probability), AFx2 (medium drop probability) and AFx3 (no drop probability). The last bit of the DSCP field is unsed and should always be 0.

The special value for the DSCP field is EF, Expedited Forwarding. EF equates to a value of 46 in the 6 bit DSCP field (101110). If you break that down, the first 3 bits (101)mean that it would be classified as CS5 and the next two (11) give it a drop probability of 3 (never drop).

The last two bits were original left unused, but have been taken over by the ECN bits.

Sorry, I realized that I didn't answer your question. Yes, devices that operate only on IP Precedence only work with the first three bits. Actually, if I remember correctly, the original IP Precedence standard did define those bits, but I don't beleive they were commonly used and the DSCP standard redefined those bits.

Hi !

Thanks a lot for your 2 two explications about DSCP, but I something in contraction in what you wrote and what I read in my book and also on Cisco Web site. I give you the reference on Cisco Web site : (on the middle of the page)

In your explication you saied : AF21 have most probability to be drop than AF23..., In my understand the last digit (in this case 1 or 3) is the probability drop value. Where the higher value mean more probability to be drop.

Your explication about the CS class is now clear for me....

It looks like your book is correct and I am wrong. Which makes the value of 46 for EF make little sense.