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Beginner

QoS assured forwarding

Hi all,

 

I am reading the QoS chapter from the new ENCOR 350-401 book and I am struggling a bit to understand the precedence of AF classes (if there is any). there is a paragraph on page 373 that says that the AF class number does not represent precedence. Than on page 374 it says that the AFxy name is composed of IP precedence in decimal.

This is where my brain started to steam :-). After all the AF class number represents precedence or not? Since, from my point of view the book is not clear on that, I am hopping that someone in this community could help me to clarify that.

 

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VIP Expert

When DSCP was RFC'ed, although it replaces IPPrec (as is reuses the same ToS "field" of the packet), its suggested implementation was done in such a way to be, more or less, backward compatible with IPPrec. So, for instance, IPPrec zero and BE are treated alike (and look alike for the IPPrec bits), IPPrecs six and seven, within DSCP, are left alone for network "stuff", and AF classes align with IPPrec 1..4 (excluding the latter RFC for scavenger, which use CS1, which has less precedence than BE although IPPrec 1 has better precedence than IPPrec 0).

So, even though one AF class does not (technically) have precedence over another, traffic suggested to be mapped to those classes assumes "older" IPPrec devices may prioritize by IPPrec value.

If this doesn't answer your question, let me know.