QOS OVERVIEW

QOS Overview

·  2 main models Integrated Services (IntServ) every flow has explicit end to end reservation rsvp is used in this not used often as does not scale well

· 2nd model is differentiated servers has a phb (per hop behaviour) traffic is grouped into classes and decision are made at each hop based on the class + queing model

· Traffic in differentiated is marked (classed) as close to the source as possible where we have  full control and user does not i.e not the user pc but maybe the ip phone or the switch

· Diferent types of marking depending on traffic type

· Different types of markings

· IPV4 & IPV6 TOS BYTE USED

      -DSCP (6 BITS)

      -IP Precedence (3 bits)

· Layer 2 markings

     Frame-relay DE bit (7 bits)

     MPLS Exp bits (3bits)

     802.1q/ISL COS bits (3 bits)

· DSCP and IP precendence are compatiable with DSCP being the newer of the two it offers more granualrity using the 6 bits of the TOS Byte IP precendence uses 3 bits

· Compatability is achieved by ip prec uses the first 3 bits for IP Prec 1-7 while DSCP uses the first 3 bits signify a forwarding class then the next 2 bits are used for an new parameter known as drop precendence so we have granualirity within the class for preferntial drops

Example

IP PREC 3

01100000  looking only at the first 3 bits 0=4 1=2 1=1 the rest are zero

DSCP

011 010 00     = AF31 

The way we work this out

The first 3 011 is = 3 so it is compatiable with ip precedence this classes as assured forwarding 3

the next 2 bits 01 equate it to be a low drop preference the last bit of 6 bits is not used

Drop preference can be 01 = low drop   10= medium drop    11= high drop

IP Precendence 1 is

00100000

while

Assured Forwarding 1 with high drop preference would be AF13

001110 00

So AF13 would drop quicker than AF11

CS1-7 are just compatiable with IP precendences the  it is dscp value with 00 drop preference i.e no drop preference so equates to same value as ip prec 1-7

To work out the DSCP decimal value from AF CLASS

The formular is 8(x) + 2 (y) where AFxy

so take

AF13

1 x 8 + 2  x 3 = 14

So AF13 = DSCP 14

FROM the DSCP DECIMAL VALUE we could work out the binary value

128    64  32  16   8   4   2   0

0         0    0    0    1    1  1    0

So we take the last 6 bits in the binary calc

01110

01= AF1 with

11= high drop 

so is AF13

·  When using MQC Match Any VS Match All  Any is any match while all it must match all the specfied matches

· MQC match commands can be used based on Access Lists, DSCP,IP Prec,NBAR,Source Int,Source/Destination Mac we can combine these also.

NBAR

· layer 4-7 inspection of traffic

· cef is required

Commands

int fa0/1

ip nbar protocol-discovery - enabling nbar on interface

sh ip nbar protocol-discovery

ip nbar port-map http tcp 80 8080 -adding custom ports