When IP traffic enters a label-switched path (LSP) tunnel, the ingress router marks all packets with a class-of-service (CoS) value, which is used to place the traffic into a transmission priority queue. On the router, each interface has up to eight transmit queues. The CoS value is encoded as part of the Multiprotocol Label Switching (MPLS) header and remains in the packets until the MPLS header is removed when the packets exit from the egress router. The routers within the LSP utilize the CoS value set at the ingress router. The CoS value is encoded by means of the CoS bits (also known as the EXP or experimental bits).
MPLS class of service works in conjunction with the router’s general CoS functionality. If you do not configure any CoS features, the default general CoS settings are used. For MPLS class of service, you might want to prioritize how the transmit queues are serviced by configuring weighted round-robin, and to configure congestion avoidance using random early detection (RED).
The next-hop label-switched router (LSR) uses the default classification shown in Table 64.
Table 64: LSR Default Classification
To configure CoS for MPLS, you can include the following statements at the [edit class-of-service] hierarchy level of the configuration.
If you do not specify a CoS value, the IP precedence bits from the packet’s IP header are used as the packet’s CoS value.
To specify a CoS value for packets in an LSP, include the class-of-service statement:
- class-of-service cos-value;
You can include this statement at the following hierarchy levels:
The class-of-service statement at the [edit protocols mpls label-switch-path] hierarchy level assigns an initial EXP value for the MPLS shim header of packets in the LSP. This value is initialized at the ingress router only and overrides the rewrite configuration established for that forwarding class. However, the CoS processing (weighted round robin [WRR] and RED) of packets entering the ingress router is not changed by the class-of-service statement on an MPLS LSP. Classification is still based on the behavior aggregate (BA) classifier at the [edit class-of-service] hierarchy level or the multifield (MF) classifier at the [edit firewall] hierarchy level.
We recommend configuring all routers along the LSP to have the same input classifier for EXP, and, if a rewrite rule is configured, all routers should have the same rewrite configuration. Otherwise, traffic at the next LSR might be classified into a different forwarding class, resulting in a different EXP value being written to the EXP header.
For more information, see the JUNOS MPLS Applications Configuration Guide.