Default Aliases for CoS Value Bit Patterns

BA classifiers use CoS values—such as DSCP, DSCP IPv6, IP precedence, IEEE 802.1, and MPLS EXP bits—to associate incoming packets with a particular CoS service level (forwarding class and packet loss priority (PLP)). You can assign a meaningful name, or alias, to the CoS values and use this alias instead of bits when configuring CoS components. These aliases are not part of the specifications but are well known through usage. For example, the alias for DSCP 101110 is widely accepted as ef (for expedited forwarding).

The 21 well-known DSCPs establish 5 DiffServ service classes:

• Best-effort (be)—The router does not apply any special CoS handling to packets with 000000 in the DiffServ field, a backward compatibility feature. There exists a high probability that these packets will be dropped under congested network conditions.

• Assured forwarding (af)—The router offers a high level of assurance that the packets are delivered as long as the packet flow from the customer stays within a certain service profile (the service provider defines the values). The router accepts excess traffic, but applies a random early detection (RED) drop profile to decide if the excess packets should be dropped and not forwarded. Three drop probabilities (low, medium, and high) are defined for this service class.

• Expedited forwarding (ef)—The router delivers assured bandwidth, low loss, low delay, and low delay variation (jitter) end-to-end for packets in this service class. Routers accept excess traffic in this class, but in contrast to assured forwarding, out-of-profile expedited-forwarding packets can be forwarded out of sequence or dropped.

• Conversational services (cs)—The router delivers assured (usually low) bandwidth with low delay and jitter for packets in this service class. Packets can be dropped, but never delivered out of sequence. Packetized voice is a good example of a conversational service.

• Network control (nc)—The router delivers packets in this service class with a low priority as these packets are not delay-sensitive. Typically, these packets represent routing protocol hello or keepalive messages and the loss of these packets jeopardizes proper network operation, so delay is preferable to discard.

Note:

CoS value aliases must begin with a letter and can be up to 64 characters long.

When you define classifiers, you can reference the CoS values by alias names. You can configure user-defined classifiers in terms of alias names. Changing the value of an alias alters the behavior of any classifier that references it.

Table 1 shows the default mappings between the CoS values and standard aliases.

Table 1: Default CoS Value Aliases

Default CoS Value Alias	CoS Value
DSCP and DSCP IPv6 CoS Aliases and CoS Values
ef	101110
af11	001010
af12	001100
af13	001110
af21	010010
af22	010100
af23	010110
af31	011010
af32	011100
af33	011110
af41	100010
af42	100100
af43	100110
be	000000
cs1	001000
cs2	010000
cs3	011000
cs4	100000
cs5	101000
nc1/cs6	110000
nc2/cs7	111000
MPLS EXP CoS Aliases and CoS Values
be	000
be1	001
ef	010
ef1	011
af11	100
af12	101
nc1/cs6	110
nc2/cs7	111
IEEE 802.1 CoS Aliases and CoS Values
be	000
be1	001
ef	010
ef1	011
af11	100
af12	101
nc1/cs6	110
nc2/cs7	111
IEEE 802.1ad CoS Aliases and CoS Values
be	0000
be-dei	0001
be1	0010
be1-dei	0011
ef	0100
ef-dei	0101
ef1	0110
ef1-dei	0111
af11	1000
af11-dei	1001
af12	1010
af12-dei	1011
nc1	1100
nc1-dei	1101
nc2	1110
nc2-dei	1111
Legacy IP Precedence CoS Aliases and CoS Values
be	000
be1	001
ef	010
ef1	011
af11	100
af12	101
nc1/cs6	110
nc2/cs7	111