Saturday, October 13, 2007

CCIE IP Precedence Vs DSCP Values

The Table bellow Represent the TOS filed in the IP Header

IPP

IPP

IPP

DropP DropP

0

ECN

ECN

Drop P = Drop Precedence IPP = IP Precedence The Drop Precedence means the probability that the packet will be dropped, the higher the value it is more likely to be dropped. IP Precedence is the importance value of a packet, the higher the value the packet is more valuable. In IP Precedence we have 0 – 7 values and you can see that by looking into the amount of fields that we have Routing (Best Effort) - 000 Priority - 001 Immediate - 010 Flash – 011 => mainly used for Voice Signaling or for Video Flash-Override - 100 Critical – 101 => mainly used for Voice RTP Internet - 110 Network - 111 You do not need to remember the name value of each one, you can simply see it with: TermServ(config)#access-list 100 permit ip any any precedence ? (0-7) Precedence value critical Match packets with critical precedence (5) flash Match packets with flash precedence (3) flash-override Match packets with flash override precedence (4) immediate Match packets with immediate precedence (2) internet Match packets with internetwork control precedence (6) network Match packets with network control precedence (7) priority Match packets with priority precedence (1) routine Match packets with routine precedence (0) with that said now we have added 2 more bits for the or more accurate 3 bits but only 2 bits we are playing with for allowing us to determine with in the scope of class witch is more preferable and for that we divided to 6 main classes Class 0 – Called also Best Effort all bits are 0 = 000000 Now we have 4 classes that are with in them divided each to 3 separate
Class1 AF11 001010 AF12 001100 AF13 001110
Class2 AF21 010010 AF22 010100 AF23 010110
Class3 AF31 011010 AF32 011100 AF33 011110
Class4 AF41 100010 AF42 100100 AF43 100110
DropP IPP Class 5 – EF Expedited Forwarding last class used for Voice Traffic Mainly 101110, and also all the DSCP values you can see with a simple ACL, but I think it is better to understand the principle and once you understand it you will not have to remember you will simply know. As it is relatively simple but the quantity makes people confused. TermServ(config)#access-list 100 permit ip any any dscp ? (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)
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