Class of Service

Support for CoS within MPLS networks (QFX5220)—Starting in Junos OS Evolved Release 22.2R1, you can use CoS within MPLS networks to prioritize certain types of traffic during periods of congestion by applying packet classifiers and rewrite rules to the MPLS traffic. We have also added MPLS EXP rewrite support.

• Default CoS on the provider (P) and provider edge (PE) routers for MPLS interfaces–The MPLS traffic uses the default EXP classifier. MPLS traffic is treated as best-effort traffic using the 802.1 default untrusted classifier. The default EXP classifier applies to all MPLS traffic on interfaces configured as family mpls. Differentiated Services code point (DSCP) classifiers are not applied to MPLS traffic.

• Default CoS on PE routers for Layer 3 interfaces–By default, all Layer 3 VPN logical interfaces are bound to default DSCP classifiers.

If you apply an EXP classifier on a penultimate-hop popping (PHP) node, then by default, the MPLS header TLL value overwrites the IP header time-to-live (TTL) value. In this case, a zero (0) overwrites the IP header DSCP bits, which signifies uniform mode. To use pipe mode, where nothing overwrites IP header TTL values or IP header DSCP bits, you should configure the following command:

set protocols mpls no-propagate-ttl

[See Understanding CoS MPLS EXP Classifiers and Rewrite Rules.]