How to Use Code Point Aliases in COS on Junos Routers

By Walter J. Goralski, Cathy Gadecki, and Michael Bushong from JUNOS OS For Dummies, 2nd Edition

Memorizing bit patterns of forwarding classes can be painful. A better way is to use code point aliases. When determining your Class of Service (CoS) policy, instead of specifying the bit pattern, you can define the alias name that represents the bit patters. Such aliases are called code point aliases.

Put simply, use code point aliases, not bit patterns. If you assign English phrases or words to a particular pattern, it’s far easier to go back and understand the configuration at a later date. Try looking at 101110; you know immediately that it refers to expedited forwarding.

Additionally, if you create a set of rules that you apply to a particular pattern and that pattern changes, changing the definition of the alias is far easier.

To simplify things even more, some well-known bit patterns already have aliases built into the software. Here are the common Differentiated Services Code Point (DSCP) aliases along with the corresponding bit patterns, forwarding classes, and PLP values.

Code Point Alias	Forwarding Class	PLP	Bit pattern
ef	expedited-forwarding	low	101110
af11	assured-forwarding	low	001010
af12	assured-forwarding	high	001100
af13	assured-forwarding	high	001110
af21	best-effort	low	010010
af22	best-effort	low	010100
af23	best-effort	low	010110
af31	best-effort	low	011010
af32	best-effort	low	011100
af33	best-effort	low	011110
af41	best-effort	low	100010
af42	best-effort	low	100100
af43	best-effort	low	100110
be	best-effort	low	000000
cs1	best-effort	low	001000
cs2	best-effort	low	010000
cs3	best-effort	low	011000
cs4	best-effort	low	100000
cs5	best-effort	low	101000
nc1/cs6	network-control	low	110000
nc2/cs7	network-control	low	111000
other	best-effort	low	(none)

These combinations aren’t exhaustive, but they represent the well-known patterns that you can leverage in your classification of traffic. If you want to add a pattern that isn’t already present, you can configure that pattern and name it whatever you want:

[edit class-of-service]
code-point aliases {
  dscp {
   my-dscp-alias 110001;
  }
}

Using this configuration, you can assign a name to a particular DSCP bit pattern. Every time you want to match on that bit pattern in the rest of the CoS configuration, you need to reference only the alias name.