From: Anthony Sequeira (terry.francona@gmail.com)
Date: Fri Dec 02 2005 - 23:18:31 GMT-3
Since there were no takers yet on my question below - I thought I might
provide the answer.....
40 is the Class Selector DSCP value that coordinates to IP Precedence 5.
Cisco decided to use the Class Selector DSCP markings in the default IP Prec
to DSCP map. I guess this was to ensure maximum compatibility.
On 12/1/05, Anthony Sequeira <terry.francona@gmail.com> wrote:
>
> The switch looks at the three left most bits of the incoming ToS field to
> determine IP Precedence. Notice it is the FIRST THREE BITS ONLY. It then
> remaps to DSCP based on your map (or the default map).
>
> The possible values for IP Precedence are:
> 000 = 0
> 001 = 1
> 010 = 2
> ...
> 111 = 7
>
> The possible values for DSCP are much wider:
> 000000 = 0
> 000001 = 1
> .....
> 111111 = 63
>
> Notice that the first three bits overlap between the two values as you
> pointed out....this is NO PROBLEM. DSCP values are fully backward
compatible
> with IP Precedence. For example - you should give voice an IP Precedence of
> 5 (101). You should give it a DSCP of 46 (101110). Notice that the first
> three bits are the same in each case!!! This is what is meant by backwards
> compatible.
>
> When you are operating within a class (for example - the assured
> forwarding classes (AF)) - you need to be careful about bigger numbers are
> better. This is not true as you found out. Bigger numbers within the class
> indicate a higher drop priority. This is a bit counter intuitive, but it is
> how it works. Notice this is only WITHIN a class, however.
>
> Notice something really strange in the default IP-DSCP map? Look at IP
> Precedence 5 - they remark it to DSCP 40!!!! What the hell? Voice is
> supposed to be marked with a DSCP value of 46 per Cisco.
>
> Who knows why they picked 40 in the default map?
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