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Configuring the MPLS CoS Bits

When traffic enters an LSP tunnel, the CoS bits in the MPLS header are set in one of two ways:

The number of the output queue into which the packet was buffered and the packet loss priority (PLP) bit are written into the MPLS header and are used as the packet’s CoS value. This behavior is the default, and no configuration is required. The JUNOS Class of Service Configuration Guide explains the IP CoS values, and summarizes how the CoS bits are treated.
You set a fixed CoS value on all packets entering the LSP tunnel. A fixed CoS value means that all packets entering the LSP receive the same class of service.

To set a fixed CoS value on all packets entering the LSP, include the class-of-service statement:




class-of-service cos-value;

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

The CoS value set using the class-of-service statement at the [edit protocols mpls] hierarchy level supersedes the CoS value set at the [edit class-of-service] hierarchy level for an interface. Effectively, the CoS value configured for an LSP overrides the CoS value set for an interface.

The CoS value can be a decimal number from 0 through 7. This number corresponds to a 3-bit binary number. The high-order 2 bits of the CoS value select which transmit queue to use on the outbound interface card.

The low-order bit of the CoS value is treated as the PLP bit and is used to select the RED drop profile to use on the output queue. If the low-order bit is 0, the non-PLP drop profile is used, and if the low-order bit is 1, the PLP drop profile is used. It is generally expected that RED will more aggressively drop packets that have the PLP bit set. For more information about RED and drop profiles, see the JUNOS Class of Service Configuration Guide.

Note: Configuring the PLP drop profile to drop packets more aggressively (for example, setting the CoS value from 6 to 7) decreases the likelihood of traffic getting through.

Table 7 summarizes how MPLS CoS values correspond to the transmit queue and PLP bit. Note that in MPLS, the mapping between the CoS bit value and the output queue is hard-coded. You cannot configure the mapping for MPLS; you can configure it only for IPv4 traffic flows, as described in the JUNOS Class of Service Configuration Guide.

Table 7: MPLS CoS Values

MPLS CoS Value	Bits	Transmit Queue	PLP Bit
0	000	0	Not set
1	001	0	Set
2	010	1	Not set
3	011	1	Set
4	100	2	Not set
5	101	2	Set
6	110	3	Not set
7	111	3	Set

Because the CoS value is part of the MPLS header, the value is associated with the packets only as they travel through the LSP tunnel. The value is not copied back to the IP header when the packets exit from the LSP tunnel.

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