CS6 / CS7 labeled?

[startx001]

Regarding  control plane packets ( cs6 and cs7 ) are they mapped to exp bits ?  are this control plane pakets labeled ?

[Seb Rupik]

Hi there,

Only the first three significant bits of the DSCP value (left-most) are copied to the MPLS EXP field. This is sufficient to capture all CS1-7 values:

cs1      dscp (001000) 
cs2      dscp (010000) 
cs3      dscp (011000) 
cs4      dscp (100000) 
cs5      dscp (101000) 
cs6      dscp (110000) 
cs7      dscp (111000)

See also:

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_classn/configuration/xe-3s/qos-classn-xe-3s-book/qos-classn-mrkg-mpls-exp.html#GUID-E0946C01-5E7A-4EBA-9F6D-45F2FE9C7138

cheers,

Seb.

[startx001]

so from what i see bits for cs6 and cs7 are also copied to exp ?

[Seb Rupik]

Yes correct, you can distinguish all seven CS values from just the first three bits.

[Joseph W. Doherty]

"Yes correct, you can distinguish all seven CS values from just the first three bits."

Not always. The DSCP CS value is all six bits. This to distinguish CS# is different from other DSCP values in same class. I.e. CS2 is different from AF2#. I.e. DSCP information can be lost.

However, as the reference Seb links to explains, you can "map" DSCP values to MPLS exp. So, if you really only wanted to retain only the CS# values, you could have the MPLS exp values represent only them. In such a case, then Seb would be correct.

What logically happens by default the MPLS exp value represents the DSCP class. For example its 0x2 would represent CS2, AF21, AF22 and AF23 (also the other bit values in that class - those not generally used).

BTW, you have the same issue when going from DSCP to IPPrec or L2 CoS, those too only have 3 bits.